Add automatic GIF preview generation for videos

Backend changes:
- Add ffmpeg to Docker image for video processing
- Create gifGenerator utility with high-quality palette-based GIF generation
- Update Prisma schema: Add GIF to MediaKind enum
- Auto-generate 300px GIF preview (10fps, 3sec) on video upload
- Add POST /api/terms/:id/media/:mediaId/regenerate-gif endpoint for admins
- Update delete endpoint to cascade delete GIFs when video is deleted
- GIF generation uses ffmpeg with palette optimization for quality

Frontend changes:
- Update MediaKind enum to include GIF
- Display animated GIF previews in dictionary word cards
- Show 'No preview' icon for videos without GIF
- Add 'Regenerate GIF' button in admin video upload UI
- Display both video and GIF side-by-side in admin panel
- Visual indicator when GIF preview is available

Features:
- Automatic GIF generation on video upload (non-blocking)
- Manual GIF regeneration from admin panel
- GIF preview in dictionary listing for better UX
- Fallback to video icon when no GIF available
- Proper cleanup: GIFs deleted when parent video is deleted

Technical details:
- GIFs stored in /uploads/gifs/
- Uses two-pass ffmpeg encoding with palette for quality
- Configurable fps, width, duration, start time
- Error handling: video upload succeeds even if GIF fails

Co-Authored-By: Auggie

packages/backend/prisma/schema.prisma

@@ -3,6 +3,7 @@
 generator client {
   provider = "prisma-client-js"
   previewFeatures = ["fullTextIndex"]
+  binaryTargets = ["native", "linux-musl-openssl-3.0.x"]
 }
 
 datasource db {
@@ -223,6 +224,7 @@ enum MediaKind {
   VIDEO
   IMAGE
   ILLUSTRATION
+  GIF
 }
 
 enum Visibility {