feat(taste): phase 4 — recommendation observability (#796)

Per-source play outcomes so the operator can see whether each recommendation
surface is landing and tune the now-operator-tunable taste weights.

Server:
- query RecommendationSourceMetricsForUser: groups the user's play_events by
  source (system-playlist surface), reporting plays / skips / avg completion
  over a window; NULL-source (library/radio) plays excluded.
- GET /api/me/recommendation-metrics?days=30 (default 30, capped 365) →
  {window_days, sources:[{source, plays, skips, skip_rate, avg_completion}]}.
- handler test: 401 unauth; per-source aggregation + NULL-source exclusion +
  skip_rate / avg_completion math.

Web:
- lib/api/metrics.ts: query + friendly source labels.
- settings page gains a "Recommendation metrics" card (table of surface / plays
  / skip rate / avg completion), with loading/error/empty states.
- settings tests mock the new query (manual subscribe-store, hoisting-safe).

Note: You-might-like plays aren't source-tagged (it's a Home row, not a system
playlist), so this covers For-You / Discover / the mixes. Tagging YML plays
would be a client follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/routes/settings/Appearance.test.ts

@@ -25,6 +25,16 @@ vi.mock('$lib/api/listenbrainz', () => {
   };
 });
 
+vi.mock('$lib/api/metrics', () => ({
+  createRecommendationMetricsQuery: () => ({
+    subscribe: (run: (v: unknown) => void) => {
+      run({ isPending: false, isError: false, data: { window_days: 30, sources: [] } });
+      return () => {};
+    }
+  }),
+  sourceLabel: (s: string) => s
+}));
+
 beforeEach(() => {
   globalThis.localStorage.clear();
   document.documentElement.removeAttribute('data-theme');