fix: add comprehensive type annotations for mypy

- database.py: add type hints for Path, Flask, Any, BaseException
- pki.py: add assertions to narrow Optional types after has_ca() checks
- routes.py: annotate config values to avoid Any return types
- api/__init__.py: use float for cleanup timestamps (time.time())
- __init__.py: remove unused return from setup_rate_limiting

app/__init__.py

@@ -174,8 +174,6 @@ def setup_rate_limiting(app: Flask) -> None:
     # Store limiter on app for use in routes
     app.extensions["limiter"] = limiter
 
-    return limiter
-
 
 def setup_metrics(app: Flask) -> None:
     """Configure Prometheus metrics."""