feat(media): expose podcast name + cover art to system Now Playing

- mpv: --force-media-title '<podcast> — <episode>' so macOS Now Playing shows
  the podcast name instead of the download-hash filename (mediaTitle PlayOption)
- media registry: artist is now the human podcast title (customName ||
  podcast.title), falling back to podcastId
- downloads: write the podcast cover as a <base>.jpg sibling so mpv's
  cover-art-auto=exact picks it up for artwork; delete it with the download

src/utils/audio-player.ts

@@ -47,6 +47,7 @@ export interface PlayOptions {
 	startPosition?: number;
 	volume?: number;
 	speed?: number;
+	mediaTitle?: string;
 }
 
 // ── Utilities ────────────────────────────────────────────────────────
@@ -109,6 +110,10 @@ export class MpvBackend implements AudioBackend {
 			`--speed=${opts?.speed ?? 1}`,
 		];
 
+		if (opts?.mediaTitle) {
+			args.push(`--force-media-title=${opts.mediaTitle}`);
+		}
+
 		if (opts?.startPosition && opts.startPosition > 0) {
 			args.push(`--start=${opts.startPosition}`);
 		}