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/services/video_presentation.py

@@ -0,0 +1,46 @@
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.models.category import Category
+from app.models.channel import Channel
+from app.models.channel_category import channel_categories
+from app.models.video import Video
+
+
+def channel_categories_map(db: Session, channel_ids: list[int]) -> dict[int, list[dict]]:
+    if not channel_ids:
+        return {}
+    rows = db.execute(
+        select(channel_categories.c.channel_id, Category.id, Category.name)
+        .join(Category, Category.id == channel_categories.c.category_id)
+        .where(channel_categories.c.channel_id.in_(channel_ids))
+    ).all()
+    result: dict[int, list[dict]] = {}
+    for channel_id, category_id, category_name in rows:
+        result.setdefault(channel_id, []).append({"id": category_id, "name": category_name})
+    return result
+
+
+def serialize_video(video: Video, channel: Channel, categories: list[dict]) -> dict:
+    return {
+        "youtube_video_id": video.youtube_video_id,
+        "title": video.title,
+        "description": video.description,
+        "channel": {
+            "id": channel.id,
+            "youtube_channel_id": channel.youtube_channel_id,
+            "title": channel.title,
+            "thumbnail_url": channel.thumbnail_url,
+        },
+        "thumbnail_url": video.thumbnail_url,
+        "published_at": video.published_at,
+        "duration_seconds": video.duration_seconds,
+        "youtube_url": video.youtube_url,
+        "categories": categories,
+        "local": {
+            "available": False,
+            "status": "not_downloaded",
+            "progress_percent": None,
+            "media_url": None,
+        },
+    }