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/queries/library_changes.sql

@@ -0,0 +1,16 @@
+-- name: InsertLibraryChange :exec
+INSERT INTO library_changes (entity_type, entity_id, op)
+VALUES ($1, $2, $3);
+
+-- name: GetLibraryChangesSince :many
+SELECT id, entity_type, entity_id, op, changed_at
+FROM library_changes
+WHERE id > $1
+ORDER BY id ASC
+LIMIT $2;
+
+-- name: GetMaxLibraryChangeCursor :one
+SELECT COALESCE(MAX(id), 0)::BIGINT FROM library_changes;
+
+-- name: GetMinLibraryChangeCursor :one
+SELECT COALESCE(MIN(id), 0)::BIGINT FROM library_changes;