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

internal/db/queries/recommendation_tuning.sql

@@ -0,0 +1,61 @@
+-- Recommendation tuning lab queries (#1250). Seeding happens via the
+-- recsettings boot reconcile; shipped defaults live in Go only.
+
+-- name: UpsertWeightProfileDefaults :exec
+-- Boot reconcile: insert the shipped defaults for a profile if the row
+-- doesn't exist yet. Never overwrites operator-tuned values.
+INSERT INTO recommendation_weight_profiles (
+    profile, base_weight, like_boost, recency_weight, skip_penalty,
+    jitter_magnitude, context_weight, similarity_weight, taste_weight
+) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+ON CONFLICT (profile) DO NOTHING;
+
+-- name: ListWeightProfiles :many
+SELECT * FROM recommendation_weight_profiles ORDER BY profile;
+
+-- name: UpdateWeightProfile :one
+UPDATE recommendation_weight_profiles
+   SET base_weight       = $2,
+       like_boost        = $3,
+       recency_weight    = $4,
+       skip_penalty      = $5,
+       jitter_magnitude  = $6,
+       context_weight    = $7,
+       similarity_weight = $8,
+       taste_weight      = $9,
+       updated_at        = now()
+ WHERE profile = $1
+RETURNING *;
+
+-- name: UpsertTasteTuningDefaults :exec
+INSERT INTO taste_tuning (
+    singleton, half_life_days, engagement_hard_skip,
+    engagement_neutral, engagement_full
+) VALUES (true, $1, $2, $3, $4)
+ON CONFLICT (singleton) DO NOTHING;
+
+-- name: GetTasteTuning :one
+SELECT * FROM taste_tuning WHERE singleton = true;
+
+-- name: UpdateTasteTuning :one
+UPDATE taste_tuning
+   SET half_life_days       = $1,
+       engagement_hard_skip = $2,
+       engagement_neutral   = $3,
+       engagement_full      = $4,
+       updated_at           = now()
+ WHERE singleton = true
+RETURNING *;
+
+-- name: InsertTuningAudit :exec
+-- changes is a jsonb array of {field, old, new} objects.
+INSERT INTO recommendation_tuning_audit (scope, action, changes)
+VALUES ($1, $2, $3);
+
+-- name: ListTuningAudit :many
+-- Newest first; consumed by the metrics trend view (#1251) to annotate
+-- knob turns on the timeline.
+SELECT id, changed_at, scope, action, changes
+  FROM recommendation_tuning_audit
+ ORDER BY changed_at DESC, id DESC
+ LIMIT $1;