feat(metrics): bucketed surface families + manual-plays baseline (#1248, milestone 127)

The recommendation metrics table was observable but not actionable: raw
source strings (album:<uuid> one-offs) drowned the stable surfaces, and
manual plays were excluded so skip rates had no control group.

- SQL: include NULL-source rows (the baseline) and carry completion_n
  so family merges can weight avg_completion correctly.
- Handler buckets raw sources into stable families (radio:<uuid> →
  Radio, album:/artist: → direct plays, etc.) grouped by surface
  intent: go-to / discovery / direct — each band judged against its
  job, since discovery mixes are expected to skip hotter. Families
  under 20 plays are flagged low-confidence, not hidden.
- Settings card renders the baseline row and per-surface deltas in
  percentage points vs baseline (worse-than-baseline deltas in danger
  color), intent hint copy per group, low-data rows dimmed.
- Pure-unit test for the bucketing/merge; DB test updated to the new
  contract (baseline included, radio:<uuid> collapse).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

web/src/lib/api/metrics.ts

@@ -1,18 +1,31 @@
 import { createQuery } from '@tanstack/svelte-query';
 import { api } from './client';
 
-// Mirrors internal/api/me_recommendation_metrics.go.
-export type RecommendationMetric = {
-  source: string;
+// Mirrors internal/api/me_recommendation_metrics.go: raw play sources are
+// bucketed server-side into stable surface families, grouped by intent, and
+// anchored by the manual-plays baseline (milestone #127).
+export type SurfaceMetric = {
+  key: string;
+  label: string;
   plays: number;
   skips: number;
   skip_rate: number;
   avg_completion: number;
+  low_confidence: boolean;
+};
+
+export type SurfaceIntent = 'go_to' | 'discovery' | 'direct';
+
+export type SurfaceGroup = {
+  intent: SurfaceIntent;
+  label: string;
+  surfaces: SurfaceMetric[];
 };
 
 export type RecommendationMetrics = {
   window_days: number;
-  sources: RecommendationMetric[];
+  baseline: SurfaceMetric | null;
+  groups: SurfaceGroup[];
 };
 
 export function getRecommendationMetrics(): Promise<RecommendationMetrics> {
@@ -28,19 +41,3 @@ export function createRecommendationMetricsQuery() {
     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;
-}