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

backend/app/services/config_service.py

@@ -200,6 +200,8 @@ async def get_jail_config(socket_path: str, name: str) -> JailConfigResponse:
         datepattern_raw,
         logencoding_raw,
         backend_raw,
+        usedns_raw,
+        prefregex_raw,
         actions_raw,
         bt_increment_raw,
         bt_factor_raw,
@@ -218,6 +220,8 @@ async def get_jail_config(socket_path: str, name: str) -> JailConfigResponse:
         _safe_get(client, ["get", name, "datepattern"], None),
         _safe_get(client, ["get", name, "logencoding"], "UTF-8"),
         _safe_get(client, ["get", name, "backend"], "polling"),
+        _safe_get(client, ["get", name, "usedns"], "warn"),
+        _safe_get(client, ["get", name, "prefregex"], ""),
         _safe_get(client, ["get", name, "actions"], []),
         _safe_get(client, ["get", name, "bantime.increment"], False),
         _safe_get(client, ["get", name, "bantime.factor"], None),
@@ -249,6 +253,8 @@ async def get_jail_config(socket_path: str, name: str) -> JailConfigResponse:
         date_pattern=str(datepattern_raw) if datepattern_raw else None,
         log_encoding=str(logencoding_raw or "UTF-8"),
         backend=str(backend_raw or "polling"),
+        use_dns=str(usedns_raw or "warn"),
+        prefregex=str(prefregex_raw) if prefregex_raw else "",
         actions=_ensure_list(actions_raw),
         bantime_escalation=bantime_escalation,
     )
@@ -329,6 +335,10 @@ async def update_jail_config(
             err = _validate_regex(pattern)
             if err:
                 raise ConfigValidationError(f"Invalid regex in {field!r}: {err!r} (pattern: {pattern!r})")
+    if update.prefregex is not None and update.prefregex:
+        err = _validate_regex(update.prefregex)
+        if err:
+            raise ConfigValidationError(f"Invalid regex in 'prefregex': {err!r} (pattern: {update.prefregex!r})")
 
     client = Fail2BanClient(socket_path=socket_path, timeout=_SOCKET_TIMEOUT)
 
@@ -356,6 +366,8 @@ async def update_jail_config(
         await _set("datepattern", update.date_pattern)
     if update.dns_mode is not None:
         await _set("usedns", update.dns_mode)
+    if update.prefregex is not None:
+        await _set("prefregex", update.prefregex)
     if update.enabled is not None:
         await _set("idle", "off" if update.enabled else "on")