feat(api): remove redundant /env and /api/diagnostic/settings endpoints

- Remove `/api/diagnostic/settings` endpoint (superseded by `/api/settings/`)
- Remove `/env` view route (superseded by `/settings` admin page)
- Delete `env_debug.html` template
- Remove `/env` nav links from base.html (desktop + mobile)
- Update status_dashboard.html to link to /settings instead of /env
- Remove corresponding tests for deleted endpoints
- Update RateLimitingStrategy.md docs

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_endpoint_registration.py

@@ -92,7 +92,6 @@ class TestEndpointRegistration:
         # Test a few known API endpoints to ensure the /api prefix works
         endpoints_to_check = [
             ("/api/process-url", "post"),
-            ("/api/diagnostic/settings", "get"),
         ]
 
         for endpoint, method in endpoints_to_check: