Implement Phases 1-5: skeleton, OAuth, categories, video sync/feed, playback

- FastAPI + PostgreSQL + Alembic + React/Vite skeleton, Docker Compose, healthcheck
- Google OAuth (single allowed account), encrypted refresh token storage
- Subscriptions sync with pagination, uploads playlist batch fetch
- Categories CRUD, many-to-many channel assignment, category filtering
- Video sync (playlistItems + videos.list batching), cached feed with cursor
  pagination, background scheduler (APScheduler)
- Video detail page with YouTube embed player
- SPA fallback routing, optimistic UI updates, client-side query caching

40 backend tests covering OAuth allow-list, sync idempotency, cascade deletes,
cursor pagination, and category filtering.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/api/videos.py

@@ -0,0 +1,24 @@
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+
+from app.core.auth_dependency import require_session
+from app.db import get_db
+from app.models.channel import Channel
+from app.models.video import Video
+from app.services.video_presentation import channel_categories_map, serialize_video
+
+router = APIRouter(dependencies=[Depends(require_session)])
+
+
+@router.get("/videos/{youtube_video_id}")
+def get_video(youtube_video_id: str, db: Session = Depends(get_db)) -> dict:
+    video = db.query(Video).filter(Video.youtube_video_id == youtube_video_id).one_or_none()
+    if video is None:
+        raise HTTPException(status_code=404, detail="Video not found")
+
+    channel = db.get(Channel, video.channel_id)
+    if channel is None:
+        raise HTTPException(status_code=404, detail="Channel not found")
+
+    categories = channel_categories_map(db, [channel.id]).get(channel.id, [])
+    return serialize_video(video, channel, categories)