feat: video support + Memories music choices

Videos:
- Worker ingests .mp4 .mov .m4v .avi .mkv .webm .3gp via ffmpeg/ffprobe
  (frame extract → sharp resize, same thumbnail pipeline as photos)
- DB: media_type + duration columns with migration
- GET /api/videos returns all indexed videos
- GET /api/photos/:id/video streams original file with Range support
  for browser seek/scrub
- VideosView: 16:9 thumbnail grid, play-button overlay, duration badge
- Full-screen player: native <video controls>, ESC to close
- Library tab now excludes videos (dedicated Videos tab)
- Dockerfile: adds ffmpeg to runtime stage

Memories music choices:
- Three styles: ⚡ Upbeat (G-major, default), 🌙 Dreamy (A-minor),
  🎬 Cinematic (D-minor, dual-oscillator richness)
- Shared Web Audio engine (reverb, LPF, pad + melody)
- Style picker in header; active style icon shown in slideshow bar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -13,7 +13,7 @@ RUN npm run build
 # ── Stage 2: runtime ─────────────────────────────────────────────────────────
 FROM node:22-alpine
 
-RUN apk add --no-cache python3 py3-pip make g++ && \
+RUN apk add --no-cache python3 py3-pip make g++ ffmpeg && \
     pip3 install icloudpd --break-system-packages
 
 WORKDIR /app