feat(server): playback_errors table + admin inbox endpoints

New client-reported playback-error log. Surfaces zero-duration
tracks (and future load_failed / stalled kinds) into an admin
inbox so the operator can hide / delete / re-request the
offending track.

Schema (migration 0032):
- playback_errors table with CHECK constraints on the kind +
  resolution enums (per the standing rule that new enum values
  need a migration to add)
- Partial index on unresolved rows for fast inbox lookup
- ON DELETE CASCADE from tracks + users so cleanup is automatic

Endpoints:
- POST /api/playback-errors: any signed-in user reports. Body
  validates track existence + kind whitelist; client_id required
  so support can correlate reports from the same device.
- GET /api/admin/playback-errors?resolved=false&offset=&limit=:
  admin list with join to track/album/artist for table render
  without per-row round-trips. Pagination capped at 200/page.
- POST /api/admin/playback-errors/{id}/resolve: admin marks
  resolved with a resolution enum string.

Auto-resolve on Hide/Delete/Re-request from the inbox row is
driven from the web client (two sequential calls) — keeps the
existing track-action endpoints unchanged.

internal/db/dbq/models.go

@@ -383,6 +383,19 @@ type PlaySession struct {
 	ClientID    *string
 }
 
+type PlaybackError struct {
+	ID         pgtype.UUID
+	TrackID    pgtype.UUID
+	UserID     pgtype.UUID
+	ClientID   string
+	Kind       string
+	Detail     *string
+	OccurredAt pgtype.Timestamptz
+	ResolvedAt pgtype.Timestamptz
+	ResolvedBy pgtype.UUID
+	Resolution *string
+}
+
 type Playlist struct {
 	ID            pgtype.UUID
 	UserID        pgtype.UUID