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/types/config.ts

@@ -50,6 +50,10 @@ export interface JailConfig {
   date_pattern: string | null;
   log_encoding: string;
   backend: string;
+  /** DNS look-up mode reported by fail2ban: "yes" | "warn" | "no" | "raw". */
+  use_dns: string;
+  /** Prefix regex prepended to every failregex; empty string means disabled. */
+  prefregex: string;
   actions: string[];
   bantime_escalation: BantimeEscalation | null;
 }
@@ -69,6 +73,8 @@ export interface JailConfigUpdate {
   find_time?: number | null;
   fail_regex?: string[] | null;
   ignore_regex?: string[] | null;
+  /** Prefix regex; undefined/null = skip, "" = clear, non-empty = set. */
+  prefregex?: string | null;
   date_pattern?: string | null;
   dns_mode?: string | null;
   enabled?: boolean | null;