Add the Secrets and Alerts screens

Two backends that had no way in: secrets could only be set through the API,
and alerting could only be configured by hand-editing its file. Both are
engine-wide operator settings rather than personal ones, so they get sidebar
entries of their own instead of tabs under the per-user Settings — and the
missing-secret error now names the place the page actually is.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

backend/app/flow/secrets.py

@@ -31,7 +31,7 @@ class SecretNotFound(KeyError):
         self.name = name
 
     def __str__(self) -> str:
-        return f"No secret named '{self.name}' — add it under Settings › Secrets."
+        return f"No secret named '{self.name}' — add it under Secrets."
 
 
 class SecretsStore: