feat(tuning): weekly trend view — per-surface series + knob-turn markers

The verify half of the tune→verify loop (#1251), on the same admin
Tuning page as the knobs:

- RecommendationWeeklyTrends: weekly per-source outcomes aggregated
  across all users (the knobs are global, so judging a turn needs
  global outcomes — rows carry rates only, no track/user identity),
  with a taste-hit count per bucket: plays whose track's artist has a
  positive weight in the player's current taste profile. That's the
  "cheap recompute" reading — retroactive over the whole window, at
  the cost of profile drift.
- GET /api/admin/recommendation-trends?weeks=N (default 12, cap 52):
  per-family weekly series (skip rate, sample-weighted completion,
  taste-hit rate) plus the tuning-audit markers inside the window.
- Web: sparkline table under the tuning cards — skip rate per week on
  a shared axis with dashed ticks at knob turns, latest-week columns,
  window taste-hit rate, low-volume rows dimmed as anecdote, and a
  plain-text list of the window's tuning changes.

Also fixes the revive unused-parameter lint on the tuning GET handler
that failed CI run 1903 on the previous commit.

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

web/src/lib/api/tuning.ts

@@ -47,3 +47,40 @@ export function patchTuning(
 export function resetTuning(scope: TuningScope): Promise<TuningSnapshot> {
   return api.post<TuningSnapshot>(`/api/admin/recommendation-tuning/${scope}/reset`, {});
 }
+
+// Trends (#1251): weekly per-surface outcome series with knob-turn
+// markers. Mirrors internal/api/admin_recommendation_trends.go.
+
+export type TrendPoint = {
+  week_start: string;
+  plays: number;
+  skips: number;
+  skip_rate: number;
+  avg_completion: number;
+  taste_hit_rate: number;
+};
+
+export type TrendSeries = {
+  key: string;
+  label: string;
+  intent: string;
+  plays: number;
+  points: TrendPoint[];
+};
+
+export type TrendMarker = {
+  changed_at: string;
+  scope: string;
+  action: string;
+  changes: { field: string; old: number; new: number }[];
+};
+
+export type TrendsResponse = {
+  weeks: number;
+  series: TrendSeries[];
+  markers: TrendMarker[];
+};
+
+export function getTrends(weeks = 12): Promise<TrendsResponse> {
+  return api.get<TrendsResponse>(`/api/admin/recommendation-trends?weeks=${weeks}`);
+}