Add mass unban: DELETE /api/bans/all clears all active bans

- Send fail2ban's `unban --all` command via new `unban_all_ips()` service
  function; returns the count of unbanned IPs
- Add `UnbanAllResponse` Pydantic model (message + count)
- Add `DELETE /api/bans/all` router endpoint; handles 502 on socket error
- Frontend: `bansAll` endpoint constant, `unbanAllBans()` API call,
  `UnbanAllResponse` type, `unbanAll` action in `useActiveBans` hook
- JailsPage: "Clear All Bans" button (visible when bans > 0) with a
  Fluent UI confirmation Dialog before executing the operation
- 7 new tests (3 service, 4 router); 440 total pass, 82% coverage

frontend/src/types/jail.ts

@@ -184,6 +184,16 @@ export interface ActiveBanListResponse {
  *
  * Mirrors `GeoDetail` from `backend/app/models/geo.py`.
  */
+/**
+ * Mirrors `UnbanAllResponse` from `backend/app/models/ban.py`.
+ */
+export interface UnbanAllResponse {
+  /** Human-readable summary of the operation. */
+  message: string;
+  /** Number of IP addresses that were unbanned. */
+  count: number;
+}
+
 export interface GeoDetail {
   /** ISO 3166-1 alpha-2 country code (e.g. `"DE"`), or `null`. */
   country_code: string | null;