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

docker-compose.yml

@@ -110,8 +110,14 @@ services:
         condition: service_completed_successfully
     environment:
       <<: *app-env
+      RUN_MIGRATIONS_ON_STARTUP: ${RUN_MIGRATIONS_ON_STARTUP:-true}
     ports:
       - "${BACKEND_PORT:-8000}:8000"
+    healthcheck:
+      test: ["CMD-SHELL", "python -c \"import urllib.request; urllib.request.urlopen('http://localhost:8000/health/ready').read()\""]
+      interval: 10s
+      timeout: 5s
+      retries: 12
 
   worker:
     build:
@@ -127,6 +133,7 @@ services:
     environment:
       <<: *app-env
       AUTO_CREATE_TABLES: false
+      RUN_MIGRATIONS_ON_STARTUP: false
 
   mailpit:
     image: axllent/mailpit:latest