Add media processing, mine feed filter, and preserve ended streams as videos

- Add ffmpeg-based media asset generation (poster, thumbnail, clip) for previews and videos
- Add GET /media/thumbnails/:filename serving route
- Add filter=mine to feed endpoint for user's own published streams
- Feed response now includes posterUrl, thumbnailUrl, clipUrl
- Deleting an ENDED plan with preview preserves it as a Video record
- Add sourcePlanId to Video schema

src/types/api.ts

@@ -149,6 +149,9 @@ export interface PublicUserResponse {
 export interface FeedItemResponse {
   plan: StreamPlanResponse & { user: { id: string; displayName: string; avatarUrl: string | null } };
   previewUrl: string | null;
+  posterUrl: string | null;
+  thumbnailUrl: string | null;
+  clipUrl: string | null;
   likeCount: number;
   commentCount: number;
   isLiked: boolean;