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>

internal/api/api.go

@@ -74,6 +74,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Post("/auth/logout", h.handleLogout)
 			authed.Get("/me", h.handleGetMe)
 			authed.Get("/me/system-playlists-status", h.handleGetSystemPlaylistsStatus)
+			authed.Get("/me/recommendation-metrics", h.handleGetRecommendationMetrics)
 			authed.Get("/me/listenbrainz", h.handleGetListenBrainz)
 			authed.Put("/me/listenbrainz", h.handlePutListenBrainz)
 			authed.Get("/me/history", h.handleGetMyHistory)