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/routes/admin/tuning/tuning.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test, vi, beforeEach } from 'vitest';
 import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
-import type { TuningSnapshot } from '$lib/api/tuning';
+import type { TuningSnapshot, TrendsResponse } from '$lib/api/tuning';
 
 vi.mock('$lib/api/tuning', async (orig) => {
   const actual = (await orig()) as Record<string, unknown>;
@@ -8,7 +8,8 @@ vi.mock('$lib/api/tuning', async (orig) => {
     ...actual,
     getTuning: vi.fn(),
     patchTuning: vi.fn(),
-    resetTuning: vi.fn()
+    resetTuning: vi.fn(),
+    getTrends: vi.fn()
   };
 });
 
@@ -16,7 +17,7 @@ const pushToast = vi.fn();
 vi.mock('$lib/stores/toast.svelte', () => ({ pushToast: (...a: unknown[]) => pushToast(...a) }));
 
 import TuningPage from './+page.svelte';
-import { getTuning, patchTuning, resetTuning } from '$lib/api/tuning';
+import { getTuning, patchTuning, resetTuning, getTrends } from '$lib/api/tuning';
 
 const weights = (over: Partial<Record<string, number>> = {}) => ({
   base_weight: 1,
@@ -50,8 +51,11 @@ function snapshot(over: Partial<TuningSnapshot> = {}): TuningSnapshot {
   } as TuningSnapshot;
 }
 
+const emptyTrends: TrendsResponse = { weeks: 12, series: [], markers: [] };
+
 beforeEach(() => {
   vi.clearAllMocks();
+  (getTrends as ReturnType<typeof vi.fn>).mockResolvedValue(emptyTrends);
 });
 
 describe('Admin tuning page', () => {
@@ -111,4 +115,80 @@ describe('Admin tuning page', () => {
       screen.getByTitle(/deviates from the shipped default \(1\.5\)/i)
     ).toBeInTheDocument();
   });
+
+  test('renders weekly trend rows with sparklines and knob-turn markers (#1251)', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    (getTrends as ReturnType<typeof vi.fn>).mockResolvedValue({
+      weeks: 12,
+      series: [
+        {
+          key: 'radio',
+          label: 'Radio',
+          intent: 'go_to',
+          plays: 40,
+          points: [
+            {
+              week_start: '2026-06-22',
+              plays: 25,
+              skips: 5,
+              skip_rate: 0.2,
+              avg_completion: 0.8,
+              taste_hit_rate: 0.6
+            },
+            {
+              week_start: '2026-06-29',
+              plays: 15,
+              skips: 6,
+              skip_rate: 0.4,
+              avg_completion: 0.7,
+              taste_hit_rate: 0.5
+            }
+          ]
+        },
+        {
+          key: 'discover',
+          label: 'Discover',
+          intent: 'discovery',
+          plays: 5,
+          points: [
+            {
+              week_start: '2026-06-29',
+              plays: 5,
+              skips: 3,
+              skip_rate: 0.6,
+              avg_completion: 0.4,
+              taste_hit_rate: 0.2
+            }
+          ]
+        }
+      ],
+      markers: [
+        {
+          changed_at: '2026-06-30T10:00:00Z',
+          scope: 'radio',
+          action: 'update',
+          changes: [{ field: 'taste_weight', old: 1, new: 2 }]
+        }
+      ]
+    } satisfies TrendsResponse);
+    render(TuningPage);
+    await waitFor(() => expect(screen.getByText('Weekly trends')).toBeInTheDocument());
+    expect(screen.getByTestId('sparkline-radio')).toBeInTheDocument();
+    expect(screen.getByTestId('sparkline-discover')).toBeInTheDocument();
+    // Latest skip rate column for radio = 40% (also discover's latest
+    // completion, hence getAllBy).
+    expect(screen.getAllByText('40%').length).toBeGreaterThan(0);
+    // The knob turn is listed under the chart.
+    expect(screen.getByText(/2026-06-30 — radio: taste_weight 1→2/)).toBeInTheDocument();
+    // Low-volume series (5 plays) is dimmed as anecdote.
+    expect(screen.getByTitle(/fewer than 20 plays in the window/i)).toBeInTheDocument();
+  });
+
+  test('empty trends show the accumulation hint', async () => {
+    (getTuning as ReturnType<typeof vi.fn>).mockResolvedValue(snapshot());
+    render(TuningPage);
+    await waitFor(() =>
+      expect(screen.getByText(/trends appear once listening accumulates/i)).toBeInTheDocument()
+    );
+  });
 });