feat(server/m7-recurring-scans): admin schedule HTTP handlers

Two new endpoints under existing RequireAdmin middleware:
- GET /api/admin/scan/schedule — returns config + computed
  next_scheduled_at (ISO 8601; null when mode='off')
- PATCH /api/admin/scan/schedule — validates per-mode required
  fields, normalizes mode='off' to NULL the per-mode fields,
  writes via UpdateScanSchedule, calls scheduler.Refresh(), returns
  the updated record.

server.New + api.Mount gain a *library.Scheduler parameter; handlers
struct + Server struct gain the scheduler field. Test stubs
(server_test.go, library_test.go's Mount call) updated to pass nil.

Tests gated on MINSTREL_TEST_DATABASE_URL: GET default is mode=off
with null next; PATCH daily produces a 03:00 next-fire; PATCH
weekly without weekly_day → 400; PATCH off normalizes lingering
per-mode fields to NULL; non-admin → 403.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/library_test.go

@@ -443,7 +443,7 @@ func TestRoutesRegisteredInMount(t *testing.T) {
 	r := chi.NewRouter()
 	w := playevents.NewWriter(h.pool, slog.New(slog.NewTextHandler(io.Discard, nil)),
 		30*time.Minute, 0.5, 30000)
-	Mount(r, h.pool, h.logger, w, config.RecommendationConfig{RadioSize: 50, RadioSizeMax: 200, RecentlyPlayedHours: 1}, h.lidarrCfg, h.lidarrRequests, h.lidarrQuarantine, h.tracks, h.playlists, h.coverart, h.coverArtBackfillCap, h.coverSettings, h.scanner, h.scanCfg, h.dataDir)
+	Mount(r, h.pool, h.logger, w, config.RecommendationConfig{RadioSize: 50, RadioSizeMax: 200, RecentlyPlayedHours: 1}, h.lidarrCfg, h.lidarrRequests, h.lidarrQuarantine, h.tracks, h.playlists, h.coverart, h.coverArtBackfillCap, h.coverSettings, h.scanner, h.scanCfg, nil, h.dataDir)
 
 	paths := []string{
 		"/api/artists",