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

backend/app/services/jail_service.py

@@ -1014,3 +1014,25 @@ async def lookup_ip(
         "currently_banned_in": currently_banned_in,
         "geo": geo,
     }
+
+
+async def unban_all_ips(socket_path: str) -> int:
+    """Unban every currently banned IP across all fail2ban jails.
+
+    Uses fail2ban's global ``unban --all`` command, which atomically removes
+    every active ban from every jail in a single socket round-trip.
+
+    Args:
+        socket_path: Path to the fail2ban Unix domain socket.
+
+    Returns:
+        The number of IP addresses that were unbanned.
+
+    Raises:
+        ~app.utils.fail2ban_client.Fail2BanConnectionError: If the socket
+            cannot be reached.
+    """
+    client = Fail2BanClient(socket_path=socket_path, timeout=_SOCKET_TIMEOUT)
+    count: int = int(_ok(await client.send(["unban", "--all"])))
+    log.info("all_ips_unbanned", count=count)
+    return count