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/routes/settings/settings.test.ts

@@ -23,11 +23,10 @@ vi.mock('$lib/api/me', () => ({
 vi.mock('$lib/api/metrics', () => ({
   createRecommendationMetricsQuery: () => ({
     subscribe: (run: (v: unknown) => void) => {
-      run({ isPending: false, isError: false, data: { window_days: 30, sources: [] } });
+      run({ isPending: false, isError: false, data: { window_days: 30, baseline: null, groups: [] } });
       return () => {};
     }
-  }),
-  sourceLabel: (s: string) => s
+  })
 }));
 
 import SettingsPage from './+page.svelte';