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- Remove `/api/diagnostic/settings` endpoint (superseded by `/api/settings/`)
- Remove `/env` view route (superseded by `/settings` admin page)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_coverage_config_settings.py

@@ -490,21 +490,6 @@ class TestLicenseRoutes:
 # ---------------------------------------------------------------------------
 
 
-class TestDiagnosticSettings:
-    """GET /api/diagnostic/settings - dump settings."""
-
-    @pytest.mark.unit
-    def test_diagnostic_settings_success(self, client):
-        """Returns safe subset of settings."""
-        with patch("app.utils.config_validator.dump_all_settings"):
-            response = client.get("/api/diagnostic/settings")
-            assert response.status_code == 200
-            data = response.json()
-            assert data["status"] == "success"
-            assert "settings" in data
-            assert "configured_services" in data["settings"]
-
-
 class TestDiagnosticTestNotification:
     """POST /api/diagnostic/test-notification - send test notification."""