fix: add type safety to PlaybackPositionService and clarify watched state logic

- Move PlaybackPositionData interface to shared-interfaces lib
- Replace all `any` types with proper PlaybackPositionData typing
- Add comment clarifying position=duration semantics for watched episodes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/playback-position.interface.ts

@@ -0,0 +1,14 @@
+/**
+ * Playback position data for resume functionality
+ */
+export interface PlaybackPositionData {
+    contentXtreamId: number;
+    contentType: 'vod' | 'episode';
+    seriesXtreamId?: number;
+    seasonNumber?: number;
+    episodeNumber?: number;
+    positionSeconds: number;
+    durationSeconds?: number;
+    playlistId?: string;
+    updatedAt?: string;
+}