Enforces config env var checks and improves startup handling (#892)

* Enforces config env var checks and improves startup handling

Ensures critical environment variables are validated during config resolution,
raising clear errors if missing. Improves server startup reliability by
verifying that backend services are listening and by terminating on
misconfiguration at launch. Adds more robust feedback to developers when
API startup fails, reducing silent misconfigurations and speeding up
troubleshooting.

* Initial plan

* Implement suggestions from PR #892: fix env var checks and improve error logging

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/src/gateway/app.py

@@ -1,9 +1,11 @@
 import logging
+import sys
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 
+from src.config.app_config import get_app_config
 from src.gateway.config import get_gateway_config
 from src.gateway.routers import artifacts, mcp, memory, models, skills, uploads
 
@@ -20,6 +22,14 @@ logger = logging.getLogger(__name__)
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     """Application lifespan handler."""
+
+    # Load config and check necessary environment variables at startup
+    try:
+        get_app_config()
+        logger.info("Configuration loaded successfully")
+    except Exception as e:
+        logger.error(f"Failed to load configuration: {e}")
+        sys.exit(1)
     config = get_gateway_config()
     logger.info(f"Starting API Gateway on {config.host}:{config.port}")