chore(prod): startup migrations, readiness checks and backend healthcheck

- add backend entrypoint that can run alembic upgrade head on startup
- add RUN_MIGRATIONS_ON_STARTUP setting and compose wiring
- add /health/live and /health/ready endpoints with db+redis checks
- add backend container healthcheck against readiness endpoint
- document readiness and startup migration behavior

Dockerfile

@@ -9,7 +9,9 @@ COPY requirements.txt .
 RUN pip install --no-cache-dir --upgrade pip && pip install --no-cache-dir -r requirements.txt
 
 COPY . .
+RUN chmod +x /app/docker/backend-entrypoint.sh
 
 EXPOSE 8000
 
+ENTRYPOINT ["/app/docker/backend-entrypoint.sh"]
 CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]