feat(#27): Gate B [07/10]: FastAPI health endpoint + app factory (H-1)

- App factory pattern (FUND-8): create_app() produces fresh app per test
- Logging config (structured, no secrets)
- Config validation: SLAB_AES_KEY required, WOO_BASE_URL HTTPS-only
- GET /api/health returns 200 {status: ok, version, woo_reachable: false}
- SQLite + alembic skeleton: full DATA-MODEL schema, initial migration
- API contract tests via httpx TestClient: 5 cases all green
  - test_returns_200, test_returns_ok_status, test_woo_reachable_false
  - test_response_shape, test_with_test_client_lifespan (FUND-9/10)

backend/requirements.txt

@@ -0,0 +1,9 @@
+fastapi>=0.115,<1
+uvicorn[standard]>=0.34,<1
+sqlalchemy>=2.0,<3
+aiosqlite>=0.20,<1
+alembic>=1.14,<2
+httpx>=0.28,<1
+pydantic>=2.9,<3
+pydantic-settings>=2.7,<3
+python-dotenv>=1.0,<2