Run health alert checks concurrently

[silentgeckoaudit3801]

Closes #206.

Summary

• check notification users concurrently with a configurable semaphore cap
• apply a per-user timeout so one slow RPC does not block the alert sweep
• record Prometheus success/failure counters for mixed alert sweep outcomes
• add async tests for failure isolation, concurrency, and timeout continuation

Validation

• Parsed the changed Python files with ast.parse using the bundled Codex Python runtime.
• Checked changed lines stay within the project lint line-length convention.
• GitHub compare reports this branch is 0 commits behind main.

Testing

• Not run as a full local project suite because this workspace is avoiding dependency installation and project toolchain execution.