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/config/app_config.py

@@ -113,7 +113,10 @@ class AppConfig(BaseModel):
         """
         if isinstance(config, str):
             if config.startswith("$"):
-                return os.getenv(config[1:], config)
+                env_value = os.getenv(config[1:])
+                if env_value is None:
+                    raise ValueError(f"Environment variable {config[1:]} not found for config value {config}")
+                return env_value
             return config
         elif isinstance(config, dict):
             return {k: cls.resolve_env_variables(v) for k, v in config.items()}