feat(server): sync.LogChange helper + EntityType/Op constants

New package internal/sync. LogChange writes a library_changes row inside
the supplied tx. Encode helpers produce stable composite ids for like_*
and playlist_track entries. Subsequent commits wire LogChange into the
scanner / likes / playlists services.

Also: dbtest.dataTables now includes library_changes so test isolation
holds across runs.

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

internal/dbtest/reset.go

@@ -54,6 +54,7 @@ var dataTables = []string{
 	"lidarr_quarantine",
 	"playlist_tracks",
 	"playlists",
+	"library_changes", // M7 #357 — must reset to keep cursor isolated per test
 	"tracks",
 	"albums",
 	"artists",