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>

migrations/versions/0003_categories.py

@@ -0,0 +1,46 @@
+"""categories and channel_categories
+
+Revision ID: 0003_categories
+Revises: 0002_oauth_channels
+Create Date: 2026-09-16
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0003_categories"
+down_revision: Union[str, None] = "0002_oauth_channels"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "categories",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+        sa.Column("name", sa.String(length=255), nullable=False),
+        sa.Column("slug", sa.String(length=255), nullable=False),
+        sa.Column("sort_order", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.UniqueConstraint("name", name="uq_categories_name"),
+        sa.UniqueConstraint("slug", name="uq_categories_slug"),
+    )
+
+    op.create_table(
+        "channel_categories",
+        sa.Column("channel_id", sa.Integer(), sa.ForeignKey("channels.id"), primary_key=True),
+        sa.Column(
+            "category_id",
+            sa.Integer(),
+            sa.ForeignKey("categories.id", ondelete="CASCADE"),
+            primary_key=True,
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("channel_categories")
+    op.drop_table("categories")