Expose usedns, date_pattern, and prefregex in jail config UI

- Add use_dns and prefregex fields to JailConfig model (backend + frontend types)
- Add prefregex to JailConfigUpdate; validate as regex before writing
- Fetch usedns and prefregex in get_jail_config via asyncio.gather
- Write usedns and prefregex in update_jail_config
- ConfigPage JailAccordionPanel: editable date_pattern input, dns_mode
  Select dropdown (yes/warn/no/raw), and prefregex input
- 8 new service unit tests + 3 new router integration tests
- 628 tests pass; 85% line coverage; ruff/mypy/tsc/eslint clean

frontend/src/components/__tests__/ConfigPageLogPath.test.tsx

@@ -116,6 +116,8 @@ const MOCK_JAIL: JailConfig = {
   date_pattern: null,
   log_encoding: "UTF-8",
   backend: "auto",
+  use_dns: "warn",
+  prefregex: "",
   actions: [],
   bantime_escalation: null,
 };