fix: sync mounted Emby favourites across web and Emby clients (#21)

Unify favourite writes through SyncUserFavorite so remote mounted media is
stored locally and proxied to the upstream Emby server. Emby clients now
query favourites from the local table (including remote pseudo IDs), and
remote item payloads merge local IsFavorite state.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/service/favorites_sync_test.go

@@ -0,0 +1,75 @@
+package service
+
+import (
+	"context"
+	"testing"
+
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+
+	"github.com/truewhile/MeBox/internal/model"
+	"github.com/truewhile/MeBox/internal/repository"
+)
+
+func TestSyncUserFavoriteWritesLocalForRemoteID(t *testing.T) {
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := db.AutoMigrate(&model.Favorite{}); err != nil {
+		t.Fatal(err)
+	}
+	repos := repository.New(db)
+	userID := "user-1"
+	remoteMediaID := EncodeEmbyRemoteID("mount-1", "remote-item-1")
+
+	if err := SyncUserFavorite(context.Background(), repos, nil, userID, remoteMediaID, true); err != nil {
+		t.Fatalf("SyncUserFavorite favorite: %v", err)
+	}
+	favorite, err := IsUserFavorite(context.Background(), repos, userID, remoteMediaID)
+	if err != nil {
+		t.Fatalf("IsUserFavorite: %v", err)
+	}
+	if !favorite {
+		t.Fatal("expected remote favourite to be stored locally")
+	}
+
+	if err := SyncUserFavorite(context.Background(), repos, nil, userID, remoteMediaID, false); err != nil {
+		t.Fatalf("SyncUserFavorite unfavorite: %v", err)
+	}
+	favorite, err = IsUserFavorite(context.Background(), repos, userID, remoteMediaID)
+	if err != nil {
+		t.Fatalf("IsUserFavorite after delete: %v", err)
+	}
+	if favorite {
+		t.Fatal("expected remote favourite to be removed locally")
+	}
+}
+
+func TestFavoriteItemsIncludesRemoteFavourites(t *testing.T) {
+	db := newServiceTestDB(t, &model.User{}, &model.Library{}, &model.Media{}, &model.Favorite{})
+	repos := repository.New(db)
+	viewer := &model.User{Username: "viewer", PasswordHash: "hash", Role: "user"}
+	if err := repos.User.Create(t.Context(), viewer); err != nil {
+		t.Fatal(err)
+	}
+	remoteMediaID := EncodeEmbyRemoteID("mount-1", "remote-item-1")
+	if err := db.Create(&model.Favorite{UserID: viewer.ID, MediaID: remoteMediaID}).Error; err != nil {
+		t.Fatal(err)
+	}
+
+	svc := &EmbyService{repo: repos}
+	out, err := svc.favoriteItems(t.Context(), ItemsParams{UserID: viewer.ID, Limit: 50})
+	if err != nil {
+		t.Fatalf("favoriteItems: %v", err)
+	}
+	total, _ := out["TotalRecordCount"].(int64)
+	if total != 0 {
+		// Without a wired remote service hydration is skipped, but local-only path
+		// should not error and should not count unavailable remote rows.
+		items, _ := out["Items"].([]map[string]any)
+		if len(items) != 0 {
+			t.Fatalf("expected no hydrated remote rows without remote service, got %#v", out)
+		}
+	}
+}