feat(ci): auto-run alembic migrations and seed on container start

- Add entrypoint.sh (migrate → seed → uvicorn)
- Add python-dotenv to requirements.txt
- Backend container uses entrypoint instead of direct uvicorn

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

scripts/entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+echo "Running Alembic migrations..."
+python -m alembic upgrade head
+
+echo "Running seed data..."
+python scripts/seed.py
+
+echo "Starting uvicorn..."
+exec uvicorn main:app --host 0.0.0.0 --port 8000