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/tuning.test.ts

@@ -0,0 +1,114 @@
+import { describe, expect, test, vi, beforeEach } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
+import type { TuningSnapshot } from '$lib/api/tuning';
+
+vi.mock('$lib/api/tuning', async (orig) => {
+  const actual = (await orig()) as Record<string, unknown>;
+  return {
+    ...actual,
+    getTuning: vi.fn(),
+    patchTuning: vi.fn(),
+    resetTuning: vi.fn()
+  };
+});
+
+const pushToast = vi.fn();
+vi.mock('$lib/stores/toast.svelte', () => ({ pushToast: (...a: unknown[]) => pushToast(...a) }));
+
+import TuningPage from './+page.svelte';
+import { getTuning, patchTuning, resetTuning } from '$lib/api/tuning';
+
+const weights = (over: Partial<Record<string, number>> = {}) => ({
+  base_weight: 1,
+  like_boost: 2,
+  recency_weight: 1,
+  skip_penalty: 2,
+  jitter_magnitude: 0.1,
+  context_weight: 0.5,
+  similarity_weight: 1.5,
+  taste_weight: 1.5,
+  ...over
+});
+
+const taste = (over: Partial<Record<string, number>> = {}) => ({
+  half_life_days: 75,
+  engagement_hard_skip: 0.05,
+  engagement_neutral: 0.3,
+  engagement_full: 0.9,
+  ...over
+});
+
+function snapshot(over: Partial<TuningSnapshot> = {}): TuningSnapshot {
+  return {
+    profiles: { radio: weights({ taste_weight: 1 }), daily_mix: weights() },
+    taste: taste(),
+    shipped: {
+      profiles: { radio: weights({ taste_weight: 1 }), daily_mix: weights() },
+      taste: taste()
+    },
+    ...over
+  } as TuningSnapshot;
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe('Admin tuning page', () => {
+  test('renders both profiles and the taste card with current values', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Radio')).toBeInTheDocument());
+    expect(screen.getByText('Daily mixes')).toBeInTheDocument();
+    expect(screen.getByText('Taste profile build')).toBeInTheDocument();
+    const radioTaste = screen.getByLabelText(/taste weight/i, {
+      selector: '#radio-taste_weight'
+    }) as HTMLInputElement;
+    expect(radioTaste.value).toBe('1');
+    const halfLife = document.getElementById('taste-half_life_days') as HTMLInputElement;
+    expect(halfLife.value).toBe('75');
+  });
+
+  test('save sends only the changed fields for the scope', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    (patchTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Radio')).toBeInTheDocument());
+    const input = document.getElementById('radio-taste_weight') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: '2.5' } });
+    await fireEvent.click(screen.getByRole('button', { name: /save radio/i }));
+    await waitFor(() =>
+      expect(patchTuning).toHaveBeenCalledWith('radio', { taste_weight: 2.5 })
+    );
+  });
+
+  test('save with no changes calls nothing and says so', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Radio')).toBeInTheDocument());
+    await fireEvent.click(screen.getByRole('button', { name: /save radio/i }));
+    expect(patchTuning).not.toHaveBeenCalled();
+    await waitFor(() => expect(pushToast).toHaveBeenCalledWith('Nothing changed.'));
+  });
+
+  test('reset calls resetTuning for the scope', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    (resetTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Taste profile build')).toBeInTheDocument());
+    const resetButtons = screen.getAllByRole('button', { name: /reset to defaults/i });
+    await fireEvent.click(resetButtons[resetButtons.length - 1]);
+    await waitFor(() => expect(resetTuning).toHaveBeenCalledWith('taste'));
+  });
+
+  test('marks knobs that deviate from shipped defaults', async () => {
+    const snap = snapshot();
+    snap.profiles.daily_mix = weights({ similarity_weight: 4 });
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snap);
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Daily mixes')).toBeInTheDocument());
+    expect(
+      screen.getByTitle(/deviates from the shipped default \(1\.5\)/i)
+    ).toBeInTheDocument();
+  });
+});