Fix restart/reload endpoint correctness and safety

- jail_service.restart(): replace invalid ["restart"] socket command with
  ["stop"], matching fail2ban transmitter protocol. The daemon is now
  stopped via socket; the caller starts it via subprocess.

- config_file_service: expose _start_daemon and _wait_for_fail2ban as
  public start_daemon / wait_for_fail2ban functions.

- restart_fail2ban router: orchestrate stop (socket) → start (subprocess)
  → probe (socket). Returns 204 on success, 503 when fail2ban does not
  come back within 10 s. Catches JailOperationError → 409.

- reload_fail2ban router: add JailOperationError catch → 409 Conflict,
  consistent with other jail control endpoints.

- Tests: add TestJailControls.test_restart_* (3 cases), TestReloadFail2ban
  502/409 cases, TestRestartFail2ban (5 cases), TestRollbackJail (6
  integration tests verifying file-write, subprocess invocation, socket-
  probe truthiness, active_jails count, and offline-at-call-time).

backend/app/services/jail_service.py

@@ -685,24 +685,29 @@ async def reload_all(
 
 
 async def restart(socket_path: str) -> None:
-    """Restart the fail2ban service (daemon).
+    """Stop the fail2ban daemon via the Unix socket.
 
-    Sends the 'restart' command to the fail2ban daemon via the Unix socket.
-    All jails are stopped and the daemon is restarted, re-reading all
-    configuration from scratch.
+    Sends ``["stop"]`` to the fail2ban daemon, which calls ``server.quit()``
+    on the daemon side and tears down all jails.  The caller is responsible
+    for starting the daemon again (e.g. via ``fail2ban-client start``).
+
+    Note:
+        ``["restart"]`` is a *client-side* orchestration command that is not
+        handled by the fail2ban server transmitter — sending it to the socket
+        raises ``"Invalid command"`` in the daemon.
 
     Args:
         socket_path: Path to the fail2ban Unix domain socket.
 
     Raises:
-        JailOperationError: If fail2ban reports the operation failed.
+        JailOperationError: If fail2ban reports the stop command failed.
         ~app.utils.fail2ban_client.Fail2BanConnectionError: If the socket
             cannot be reached.
     """
     client = Fail2BanClient(socket_path=socket_path, timeout=_SOCKET_TIMEOUT)
     try:
-        _ok(await client.send(["restart"]))
-        log.info("fail2ban_restarted")
+        _ok(await client.send(["stop"]))
+        log.info("fail2ban_stopped_for_restart")
     except ValueError as exc:
         raise JailOperationError(str(exc)) from exc