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/db/queries/recommendation_metrics.sql

@@ -0,0 +1,22 @@
+-- Recommendation observability (#796 phase 4). Per-source play outcomes so the
+-- operator can see whether each recommendation surface is landing and tune the
+-- taste weights. Source is stamped on play_events when a play is launched from
+-- a system-playlist surface ('for_you' | 'discover' | the discovery mixes);
+-- NULL for library / radio / user-playlist plays, which are excluded here.
+
+-- name: RecommendationSourceMetricsForUser :many
+-- $1 user_id, $2 window_days. plays/skips are counts; avg_completion is the
+-- mean completion ratio over plays that recorded one (0 when none did).
+SELECT
+    pe.source,
+    count(*)::bigint AS plays,
+    count(*) FILTER (WHERE pe.was_skipped)::bigint AS skips,
+    COALESCE(
+        avg(pe.completion_ratio) FILTER (WHERE pe.completion_ratio IS NOT NULL),
+        0)::float8 AS avg_completion
+FROM play_events pe
+WHERE pe.user_id = $1
+  AND pe.source IS NOT NULL
+  AND pe.started_at > now() - ($2::float8 * INTERVAL '1 day')
+GROUP BY pe.source
+ORDER BY plays DESC;