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/lib/api/metrics.ts

@@ -0,0 +1,46 @@
+import { createQuery } from '@tanstack/svelte-query';
+import { api } from './client';
+
+// Mirrors internal/api/me_recommendation_metrics.go.
+export type RecommendationMetric = {
+  source: string;
+  plays: number;
+  skips: number;
+  skip_rate: number;
+  avg_completion: number;
+};
+
+export type RecommendationMetrics = {
+  window_days: number;
+  sources: RecommendationMetric[];
+};
+
+export function getRecommendationMetrics(): Promise<RecommendationMetrics> {
+  return api.get<RecommendationMetrics>('/api/me/recommendation-metrics');
+}
+
+export const REC_METRICS_QUERY_KEY = ['settings', 'recommendation-metrics'] as const;
+
+export function createRecommendationMetricsQuery() {
+  return createQuery({
+    queryKey: REC_METRICS_QUERY_KEY,
+    queryFn: getRecommendationMetrics,
+    staleTime: 60_000
+  });
+}
+
+// Friendly labels for the system-playlist source keys (play_events.source).
+const SOURCE_LABELS: Record<string, string> = {
+  for_you: 'For You',
+  discover: 'Discover',
+  deep_cuts: 'Deep cuts',
+  rediscover: 'Rediscover',
+  new_for_you: 'New for you',
+  on_this_day: 'On this day',
+  first_listens: 'First listens',
+  songs_like_artist: 'Songs like…'
+};
+
+export function sourceLabel(source: string): string {
+  return SOURCE_LABELS[source] ?? source;
+}