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/sync/changes.go

@@ -0,0 +1,42 @@
+package sync
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// LogChange writes a row to library_changes inside the supplied transaction.
+// Callers MUST pass the same tx that performed the underlying mutation —
+// the change row and the mutation must commit or roll back together.
+//
+// entityID is the string form of the row's primary key. For composite-key
+// entities (likes, playlist_tracks), use EncodeLikeID / EncodePlaylistTrackID
+// to produce stable strings.
+func LogChange(ctx context.Context, tx pgx.Tx, entityType EntityType, entityID string, op Op) error {
+	q := dbq.New(tx)
+	if err := q.InsertLibraryChange(ctx, dbq.InsertLibraryChangeParams{
+		EntityType: string(entityType),
+		EntityID:   entityID,
+		Op:         string(op),
+	}); err != nil {
+		return fmt.Errorf("sync.LogChange(%s, %s, %s): %w", entityType, entityID, op, err)
+	}
+	return nil
+}
+
+// EncodeLikeID joins a user UUID and an entity UUID into the stable
+// composite identifier used for like_* rows in library_changes. Both
+// sides are UUID strings so no escaping is needed.
+func EncodeLikeID(userID, entityID string) string {
+	return userID + ":" + entityID
+}
+
+// EncodePlaylistTrackID joins a playlist UUID and a track UUID into the
+// stable composite identifier used for playlist_track rows.
+func EncodePlaylistTrackID(playlistID, trackID string) string {
+	return playlistID + ":" + trackID
+}