feat: M1 foundation — compose stack, FastAPI, Google OAuth, encrypted tokens

- docker-compose with Postgres 16 + slim Python API image
- FastAPI app with session middleware, health endpoint, static login page
- Google OAuth (Authlib) with email invite-list whitelist; admin role support
- User + OAuthToken models; refresh tokens encrypted at rest (Fernet)
- Alembic migrations, run automatically on container startup
- Postgres backup/restore scripts for portability between machines

backend/entrypoint.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -e
+
+echo "Applying database migrations..."
+alembic upgrade head
+
+echo "Starting Subfeed API..."
+exec uvicorn app.main:app --host 0.0.0.0 --port 8000