feat(server): library_changes log table + sqlc queries

Append-only change log for library entities. Every mutation on
artists/albums/tracks/likes/playlists/playlist_tracks will write a row
in the same transaction as the mutation itself (wired in subsequent
commits). Powers the Flutter delta-sync endpoint (#357).

- 0025_library_changes migration (up + down)
- internal/db/queries/library_changes.sql (Insert, GetSince, MaxCursor, MinCursor)
- regenerated dbq from sqlc

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/migrations/0025_library_changes.up.sql

@@ -0,0 +1,23 @@
+-- Append-only change log for library entities. Every mutation on
+-- artists/albums/tracks/likes/playlists/playlist_tracks writes a row
+-- in the same transaction as the mutation itself. Powers the Flutter
+-- delta-sync endpoint (#357) — clients pass back the last-seen `id`
+-- as a cursor.
+--
+-- Retention: not capped here. If the table grows unbounded a follow-up
+-- adds a periodic compactor (collapse contiguous upserts on the same
+-- entity, drop pre-delete history). Cursor reset path (410) covers the
+-- compactor's edge.
+
+CREATE TABLE library_changes (
+  id BIGSERIAL PRIMARY KEY,
+  entity_type TEXT NOT NULL CHECK (entity_type IN (
+    'artist', 'album', 'track', 'like_track', 'like_album', 'like_artist',
+    'playlist', 'playlist_track'
+  )),
+  entity_id TEXT NOT NULL,
+  op TEXT NOT NULL CHECK (op IN ('upsert', 'delete')),
+  changed_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_library_changes_id ON library_changes(id);