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/migrations/0032_playback_errors.up.sql

@@ -0,0 +1,41 @@
+-- Client-reported playback errors. Populated by clients when a track
+-- fails to play (zero duration, decode error, etc.); surfaced in the
+-- admin /admin/playback-errors inbox so the operator can hide / delete
+-- / re-request the offending track.
+--
+-- resolved_at + resolved_by + resolution are NULL until an admin acts
+-- on the row. Auto-resolved by the client when the admin clicks Hide /
+-- Delete / Re-request from the inbox row, with the resolution string
+-- recording which path was taken. The partial index keeps the
+-- unresolved-queue lookup fast even as resolved history accumulates.
+--
+-- CHECK constraints on kind/resolution gate the enum values so client
+-- typos can't pollute the data (per the standing rule about
+-- enum-CHECK whitelists needing migrations).
+
+CREATE TABLE playback_errors (
+    id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    track_id    uuid NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    user_id     uuid NOT NULL REFERENCES users(id)  ON DELETE CASCADE,
+    client_id   text NOT NULL,
+    kind        text NOT NULL,
+    detail      text,
+    occurred_at timestamptz NOT NULL DEFAULT now(),
+    resolved_at timestamptz,
+    resolved_by uuid REFERENCES users(id),
+    resolution  text,
+    CONSTRAINT playback_errors_kind_check
+        CHECK (kind IN ('zero_duration', 'load_failed', 'stalled')),
+    CONSTRAINT playback_errors_resolution_check
+        CHECK (resolution IS NULL OR resolution IN
+            ('hidden', 'deleted', 'requested', 'fixed', 'ignored')),
+    CONSTRAINT playback_errors_resolved_consistency
+        CHECK ((resolved_at IS NULL) = (resolution IS NULL))
+);
+
+CREATE INDEX idx_playback_errors_unresolved
+    ON playback_errors (occurred_at DESC)
+ WHERE resolved_at IS NULL;
+
+CREATE INDEX idx_playback_errors_track
+    ON playback_errors (track_id);