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/app/main.py

@@ -0,0 +1,49 @@
+"""App factory and lifespan."""
+
+import logging
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+
+from app.api import health
+from app.config import Settings
+from app.db.engine import close_db_connection, init_db
+from app.logging_config import configure_logging
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Application lifespan: startup and shutdown hooks.
+
+    Handles database initialization, logging configuration,
+    and cleanup on shutdown.
+    """
+    configure_logging()
+    logging.info("SlabUploader starting up")
+    await init_db()
+    yield
+    logging.info("SlabUploader shutting down")
+    await close_db_connection()
+
+
+def create_app(settings: Settings | None = None) -> FastAPI:
+    """Create a fresh FastAPI app instance.
+
+    Each call produces an independent app with its own database connection
+    and cleared dependency overrides. This is the factory used by tests.
+
+    Args:
+        settings: Application settings. Uses env vars if None.
+
+    Returns:
+        Configured FastAPI application.
+    """
+    if settings is None:
+        settings = Settings()
+    settings.validate()
+
+    app = FastAPI(title="SlabUploader", version="0.1.0", lifespan=lifespan)
+
+    app.include_router(health.router)
+
+    return app