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

prisma/schema.prisma

@@ -166,6 +166,7 @@ model Video {
   videoUrl     String
   fileSize     Int?
   isPublic     Boolean     @default(true)
+  sourcePlanId String?
   createdAt    DateTime    @default(now())
   updatedAt    DateTime    @updatedAt
   likes        VideoLike[]