Split TrackApplicationResponse into a Local/External hierarchy

Same treatment as AlbumApplicationResponse, but Track is the cleaner
case: OwnerUserId (local-only) and ExternalId (external-only) are
both genuinely exclusive to one side, so the base carries neither and
is abstract — every real value is one of the two derived types.

    TrackApplicationResponse (abstract, shared fields + FromEntity dispatch)
      LocalTrackApplicationResponse    (+ OwnerUserId)
      ExternalTrackApplicationResponse (+ ExternalId)

Hit a real gotcha along the way: the response already had an explicit
Source property, which collides with the "source" discriminator STJ's
polymorphism support injects ("contains property 'source' that
conflicts with an existing metadata property name" — only surfaced at
runtime in the Api integration tests, not at build time). Fixed by
dropping the now-redundant explicit Source field entirely; the
discriminator already serializes the identical "Local"/"YouTube"
value under the same name, so the wire contract is unchanged. Updated
the two call sites that read .Source directly (TrackSearchItemResponse
and a playlist track filter that needed flatMap instead of
filter().map() to keep the narrowing across the chain).

web-player/src/lib/api/schema.d.ts

@@ -549,7 +549,34 @@ export interface components {
             source: null | components["schemas"]["TrackSource"];
             externalId: null | string;
         };
-        AlbumApplicationResponse: {
+        AlbumApplicationResponse: components["schemas"]["AlbumApplicationResponseAlbumApplicationResponse"] | components["schemas"]["AlbumApplicationResponseExternalAlbumApplicationResponse"];
+        AlbumApplicationResponseAlbumApplicationResponse: {
+            /** @enum {string} */
+            source?: "Local";
+            /** Format: uuid */
+            id: string;
+            title: string;
+            description: null | string;
+            /** Format: int32 */
+            releaseYear: null | number | string;
+            /** Format: int64 */
+            ownerUserId: number | string;
+            /** Format: int32 */
+            trackCount: number | string;
+            smallImageKeyName: null | string;
+            mediumImageKeyName: null | string;
+            largeImageKeyName: null | string;
+            /** Format: date-time */
+            createdAt: string;
+            /** Format: date-time */
+            updatedAt: string;
+            coverTrackIds: string[];
+        };
+        AlbumApplicationResponseExternalAlbumApplicationResponse: {
+            /** @enum {string} */
+            source?: "YouTube";
+            externalId: string;
+            thumbnailUrl: null | string;
             /** Format: uuid */
             id: string;
             title: string;
@@ -568,8 +595,6 @@ export interface components {
             /** Format: date-time */
             updatedAt: string;
             coverTrackIds: string[];
-            youTubeAlbumId?: null | string;
-            thumbnailUrl?: null | string;
         };
         AlbumPictureUploadResponse: {
             /** Format: uuid */
@@ -766,20 +791,39 @@ export interface components {
             /** Format: int64 */
             expectedAudioSizeBytes?: null | number | string;
         };
-        TrackApplicationResponse: {
+        TrackApplicationResponse: components["schemas"]["TrackApplicationResponseLocalTrackApplicationResponse"] | components["schemas"]["TrackApplicationResponseExternalTrackApplicationResponse"];
+        TrackApplicationResponseExternalTrackApplicationResponse: {
+            /** @enum {string} */
+            source?: "YouTube";
+            externalId: string;
             /** Format: uuid */
             id: string;
             title: string;
             artist: null | string;
-            source: components["schemas"]["TrackSource"];
-            externalId: null | string;
             audioStatus: components["schemas"]["ProcessingStatus"];
             /** Format: double */
             duration: number | string;
             /** Format: int32 */
             listensCount: number | string;
+            /** Format: date-time */
+            createdAt: string;
+            /** Format: date-time */
+            updatedAt: string;
+        };
+        TrackApplicationResponseLocalTrackApplicationResponse: {
+            /** @enum {string} */
+            source?: "Local";
             /** Format: int64 */
-            ownerUserId: null | number | string;
+            ownerUserId: number | string;
+            /** Format: uuid */
+            id: string;
+            title: string;
+            artist: null | string;
+            audioStatus: components["schemas"]["ProcessingStatus"];
+            /** Format: double */
+            duration: number | string;
+            /** Format: int32 */
+            listensCount: number | string;
             /** Format: date-time */
             createdAt: string;
             /** Format: date-time */

web-player/src/routes/(app)/playlists/[id]/+page.server.ts

@@ -40,7 +40,7 @@ export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
 	const library = (libraryRes.data?.items ?? []).filter((t) => !inPlaylist.has(t.id));
 
 	const alreadyLinked = new Set(
-		tracks.filter((t) => t.source === 'YouTube' && t.externalId).map((t) => t.externalId)
+		tracks.flatMap((t) => (t.source === 'YouTube' && t.externalId ? [t.externalId] : []))
 	);
 	const seeds = [...new Set(tracks.map((t) => t.artist).filter((a): a is string => !!a))];
 	const seed = seeds.length > 0 ? shuffle(seeds)[0] : undefined;