feat(docker): add all-in-one Dockerfile without Playwright (<200MB target)

Multi-stage build:
- Stage 1: node:20-alpine builds frontend (discarded)
- Stage 2: python:3.11-slim runs backend + serves SPA static files
- Playwright removed from deps (saves ~350MB)
- Uvicorn serves both API and frontend on :8000
- Healthcheck via curl
- Entrypoint with admin bootstrap support

Estimated image size: ~190MB (vs ~550MB with playwright)

backend/requirements-docker.txt

@@ -0,0 +1,57 @@
+annotated-doc==0.0.4
+annotated-types==0.7.0
+anyio==4.12.0
+APScheduler==3.11.2
+attrs==25.4.0
+Authlib==1.6.6
+certifi==2025.11.12
+cffi==2.0.0
+charset-normalizer==3.4.4
+click==8.3.1
+cryptography==46.0.3
+fastapi==0.126.0
+greenlet==3.3.0
+h11==0.16.0
+httpcore==1.0.9
+httpx==0.28.1
+idna==3.11
+jaraco.classes==3.4.0
+jaraco.context==6.0.1
+jaraco.functools==4.3.0
+jeepney==0.9.0
+jsonschema==4.25.1
+jsonschema-specifications==2025.9.1
+keyring==25.7.0
+more-itertools==10.8.0
+pycparser==2.23
+pydantic==2.12.5
+pydantic-settings
+pydantic_core==2.41.5
+python-multipart==0.0.21
+PyYAML==6.0.3
+passlib[bcrypt]
+python-jose[cryptography]
+PyJWT
+RapidFuzz==3.14.3
+referencing==0.37.0
+requests==2.32.5
+rpds-py==0.30.0
+SecretStorage==3.5.0
+SQLAlchemy==2.0.45
+starlette==0.50.0
+typing-inspection==0.4.2
+typing_extensions==4.15.0
+tzlocal==5.3.1
+urllib3==2.6.2
+uvicorn==0.38.0
+websockets==15.0.1
+pandas
+psycopg2-binary
+openpyxl
+GitPython==3.1.44
+itsdangerous
+email-validator
+openai
+tenacity
+Pillow
+ruff>=0.11.0