feat(tuning): scoring weights → DB-backed admin tuning lab

The recommendation scoring knobs move out of YAML (radio profile) and
out of the systemMixWeights hard-code (daily_mix profile) into
DB-backed settings with live effect (#1250) — the defaults-discovery
lab per decision #1247: the operator turns knobs to find good values,
which then get baked back into shipped defaults; end users and other
operators should never need the card.

- Migration 0040: recommendation_weight_profiles (radio / daily_mix,
  8 weight columns), taste_tuning singleton (engagement half-life +
  completion-curve points), recommendation_tuning_audit (one row per
  change with a {field, old, new} diff — the trend view's markers,
  #1251).
- internal/recsettings: boot reconcile seeds shipped defaults without
  clobbering tuned rows (coverart SettingsService pattern), validates
  patches (bounds, curve ordering), writes audit rows, and pushes
  daily_mix weights + taste config into package playlists. No-op
  patches write no audit row.
- playlists gains SetSystemMixWeights / SetTasteConfig swap points
  under a RWMutex — no signature threading through the producers; the
  scheduler's taste rebuild reads the pushed config.
- Radio reads its weight profile from the service per request; the 8
  weight fields leave config.RecommendationConfig (YAML keeps only
  RecentlyPlayedHours / RadioSize / RadioSizeMax).
- Admin API: GET/PATCH/reset under /api/admin/recommendation-tuning,
  echoing current + shipped values.
- Web: new admin Tuning tab — two weight profiles side by side, taste
  card, per-scope save (changed fields only) + reset, deviation dots
  against shipped defaults.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

web/src/routes/admin/tuning/+page.svelte

@@ -0,0 +1,250 @@
+<script lang="ts">
+  import { pageTitle } from '$lib/branding';
+  import {
+    getTuning,
+    patchTuning,
+    resetTuning,
+    type TuningScope,
+    type TuningSnapshot,
+    type WeightProfile,
+    type TasteTuning
+  } from '$lib/api/tuning';
+  import { errMessage } from '$lib/api/errors';
+  import { pushToast } from '$lib/stores/toast.svelte';
+
+  // The defaults-discovery lab (#1250): DB-backed scoring-weight
+  // profiles + taste-build knobs with live effect. This card exists to
+  // FIND good values — found-good values get baked into shipped
+  // defaults, so end users and other operators never need it.
+
+  const weightFields: { key: keyof WeightProfile; label: string; hint: string }[] = [
+    { key: 'base_weight',       label: 'Base weight',       hint: 'Floor score every candidate starts from.' },
+    { key: 'like_boost',        label: 'Like boost',        hint: 'Added when the track is liked.' },
+    { key: 'recency_weight',    label: 'Recency weight',    hint: 'Rewards tracks not played recently (0–30d ramp).' },
+    { key: 'skip_penalty',      label: 'Skip penalty',      hint: 'Subtracts skips/plays ratio.' },
+    { key: 'jitter_magnitude',  label: 'Jitter',            hint: 'Random reshuffle magnitude for near-ties.' },
+    { key: 'context_weight',    label: 'Context weight',    hint: 'Session-vector similarity contribution.' },
+    { key: 'similarity_weight', label: 'Similarity weight', hint: 'Seed-similarity contribution.' },
+    { key: 'taste_weight',      label: 'Taste weight',      hint: 'Learned taste-profile fit, in [-1, +1].' }
+  ];
+
+  const tasteFields: { key: keyof TasteTuning; label: string; hint: string }[] = [
+    { key: 'half_life_days',       label: 'Half-life (days)', hint: "A play's influence halves every this-many days." },
+    { key: 'engagement_hard_skip', label: 'Hard-skip point',  hint: 'Completion at/below which a play reads −1.' },
+    { key: 'engagement_neutral',   label: 'Neutral point',    hint: 'Completion at which a play reads 0.' },
+    { key: 'engagement_full',      label: 'Full point',       hint: 'Completion at/above which a play reads +1.' }
+  ];
+
+  const profileScopes: { scope: 'radio' | 'daily_mix'; label: string; blurb: string }[] = [
+    { scope: 'radio', label: 'Radio', blurb: 'Seed-directed listening — the user picked a direction.' },
+    { scope: 'daily_mix', label: 'Daily mixes', blurb: 'For You, Songs like…, and the discovery mixes.' }
+  ];
+
+  let snapshot = $state<TuningSnapshot | null>(null);
+  let loadFailed = $state(false);
+  // Editable copies, string-typed for the inputs; parsed on save.
+  let form = $state<Record<string, Record<string, string>>>({});
+  let saving = $state<TuningScope | null>(null);
+
+  function fillForm(snap: TuningSnapshot) {
+    const f: Record<string, Record<string, string>> = { radio: {}, daily_mix: {}, taste: {} };
+    for (const p of ['radio', 'daily_mix'] as const) {
+      for (const { key } of weightFields) f[p][key] = String(snap.profiles[p][key]);
+    }
+    for (const { key } of tasteFields) f.taste[key] = String(snap.taste[key]);
+    form = f;
+  }
+
+  $effect(() => {
+    getTuning()
+      .then((snap) => {
+        fillForm(snap);
+        snapshot = snap;
+      })
+      .catch(() => {
+        loadFailed = true;
+      });
+  });
+
+  // A knob deviates when its CURRENT SAVED value differs from shipped;
+  // the dot marks where this install has drifted from defaults.
+  function deviates(scope: 'radio' | 'daily_mix' | 'taste', key: string): boolean {
+    if (!snapshot) return false;
+    if (scope === 'taste') {
+      return snapshot.taste[key as keyof TasteTuning] !== snapshot.shipped.taste[key as keyof TasteTuning];
+    }
+    return (
+      snapshot.profiles[scope][key as keyof WeightProfile] !==
+      snapshot.shipped.profiles[scope][key as keyof WeightProfile]
+    );
+  }
+
+  function currentValue(scope: TuningScope, key: string): number {
+    if (!snapshot) return 0;
+    if (scope === 'taste') return snapshot.taste[key as keyof TasteTuning];
+    return snapshot.profiles[scope][key as keyof WeightProfile];
+  }
+
+  async function save(scope: TuningScope) {
+    if (!snapshot) return;
+    const values: Record<string, number> = {};
+    for (const [key, raw] of Object.entries(form[scope] ?? {})) {
+      const v = Number(raw);
+      if (!Number.isFinite(v)) {
+        pushToast(`${key} is not a number.`, 'error');
+        return;
+      }
+      if (v !== currentValue(scope, key)) values[key] = v;
+    }
+    if (Object.keys(values).length === 0) {
+      pushToast('Nothing changed.');
+      return;
+    }
+    saving = scope;
+    try {
+      snapshot = await patchTuning(scope, values);
+      fillForm(snapshot);
+      pushToast('Saved — takes effect on the next scoring pass.');
+    } catch (e: unknown) {
+      pushToast(`Save failed: ${errMessage(e)}`, 'error');
+    } finally {
+      saving = null;
+    }
+  }
+
+  async function reset(scope: TuningScope) {
+    saving = scope;
+    try {
+      snapshot = await resetTuning(scope);
+      fillForm(snapshot);
+      pushToast('Reset to shipped defaults.');
+    } catch (e: unknown) {
+      pushToast(`Reset failed: ${errMessage(e)}`, 'error');
+    } finally {
+      saving = null;
+    }
+  }
+</script>
+
+<svelte:head>
+  <title>{pageTitle('Tuning')}</title>
+</svelte:head>
+
+<div class="space-y-6 p-4">
+  <div>
+    <h1 class="text-xl font-semibold">Recommendation tuning</h1>
+    <p class="mt-1 max-w-3xl text-sm text-text-secondary">
+      The defaults-discovery lab. Changes apply live — radio on the next request, daily mixes on
+      the next rebuild — and every change is recorded so the metrics page can tie outcome shifts
+      to knob turns. Found-good values get baked into shipped defaults; a dot marks knobs that
+      currently deviate from them.
+    </p>
+  </div>
+
+  {#if loadFailed}
+    <p class="text-sm text-text-secondary">Couldn't load tuning settings.</p>
+  {:else if !snapshot}
+    <p class="text-sm text-text-secondary">Loading…</p>
+  {:else}
+    <section class="grid gap-4 lg:grid-cols-2">
+      {#each profileScopes as p (p.scope)}
+        <div class="space-y-3 rounded border border-border bg-surface p-4">
+          <div>
+            <h2 class="text-lg font-semibold">{p.label}</h2>
+            <p class="text-xs text-text-secondary">{p.blurb}</p>
+          </div>
+          <div class="space-y-2">
+            {#each weightFields as f (f.key)}
+              <div class="grid grid-cols-[1fr_7rem] items-center gap-2">
+                <label class="text-sm" for="{p.scope}-{f.key}" title={f.hint}>
+                  {f.label}
+                  {#if deviates(p.scope, f.key)}
+                    <span
+                      class="ml-1 inline-block h-1.5 w-1.5 rounded-full bg-accent align-middle"
+                      title="Deviates from the shipped default ({snapshot.shipped.profiles[p.scope][f.key]})"
+                    ></span>
+                  {/if}
+                </label>
+                <input
+                  id="{p.scope}-{f.key}"
+                  type="number"
+                  step="0.1"
+                  bind:value={form[p.scope][f.key]}
+                  class="w-full rounded border border-border bg-background px-2 py-1 text-right text-sm tabular-nums outline-none focus:border-accent"
+                />
+              </div>
+            {/each}
+          </div>
+          <div class="flex gap-2">
+            <button
+              type="button"
+              disabled={saving !== null}
+              onclick={() => save(p.scope)}
+              class="rounded bg-accent px-3 py-1.5 text-sm text-white disabled:opacity-50"
+            >
+              Save {p.label.toLowerCase()}
+            </button>
+            <button
+              type="button"
+              disabled={saving !== null}
+              onclick={() => reset(p.scope)}
+              class="rounded border border-border px-3 py-1.5 text-sm text-text-secondary hover:text-text-primary disabled:opacity-50"
+            >
+              Reset to defaults
+            </button>
+          </div>
+        </div>
+      {/each}
+    </section>
+
+    <section class="space-y-3 rounded border border-border bg-surface p-4 lg:max-w-xl">
+      <div>
+        <h2 class="text-lg font-semibold">Taste profile build</h2>
+        <p class="text-xs text-text-secondary">
+          How plays become the learned taste profile: the influence half-life and the
+          completion→engagement curve (must stay ordered: hard-skip &lt; neutral &lt; full).
+        </p>
+      </div>
+      <div class="space-y-2">
+        {#each tasteFields as f (f.key)}
+          <div class="grid grid-cols-[1fr_7rem] items-center gap-2">
+            <label class="text-sm" for="taste-{f.key}" title={f.hint}>
+              {f.label}
+              {#if deviates('taste', f.key)}
+                <span
+                  class="ml-1 inline-block h-1.5 w-1.5 rounded-full bg-accent align-middle"
+                  title="Deviates from the shipped default ({snapshot.shipped.taste[f.key]})"
+                ></span>
+              {/if}
+            </label>
+            <input
+              id="taste-{f.key}"
+              type="number"
+              step={f.key === 'half_life_days' ? '1' : '0.05'}
+              bind:value={form.taste[f.key]}
+              class="w-full rounded border border-border bg-background px-2 py-1 text-right text-sm tabular-nums outline-none focus:border-accent"
+            />
+          </div>
+        {/each}
+      </div>
+      <div class="flex gap-2">
+        <button
+          type="button"
+          disabled={saving !== null}
+          onclick={() => save('taste')}
+          class="rounded bg-accent px-3 py-1.5 text-sm text-white disabled:opacity-50"
+        >
+          Save taste
+        </button>
+        <button
+          type="button"
+          disabled={saving !== null}
+          onclick={() => reset('taste')}
+          class="rounded border border-border px-3 py-1.5 text-sm text-text-secondary hover:text-text-primary disabled:opacity-50"
+        >
+          Reset to defaults
+        </button>
+      </div>
+    </section>
+  {/if}
+</div>