plugin: watchtower: call start_watching() threadsafe

Launch `WatchtowerPlugin.watchtower.start_watching()` through
asyncio.run_coroutine_threadsafe instead of ensure_future to prevent the
following exception from happening when running python with
`PYTHONASYNCIODEBUG=1`.

```

20251103T165007.087746Z |    ERROR | plugin.Plugins | cannot initialize plugin watchtower: Error loading watchtower plugin: RuntimeError('Non-thread-safe operation invoked on an event loop other than the current one')
Traceback (most recent call last):
  File "/home/user/code/electrum-fork/electrum/plugin.py", line 629, in load_plugin_by_name
    plugin = module.Plugin(self, self.config, name)
  File "/home/user/code/electrum-fork/electrum/plugins/watchtower/watchtower.py", line 59, in __init__
    asyncio.ensure_future(self.watchtower.start_watching())
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/asyncio/tasks.py", line 732, in ensure_future
    return loop.create_task(coro_or_future)
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/asyncio/base_events.py", line 468, in create_task
    return self._task_factory(self, coro, **kwargs)
           ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/code/electrum-fork/electrum/util.py", line 1773, in factory
    task = asyncio.Task(coro, loop=loop_, **kwargs)
  File "/usr/lib64/python3.14/asyncio/base_events.py", line 829, in call_soon
    self._check_thread()
    ~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/asyncio/base_events.py", line 866, in _check_thread
    raise RuntimeError(
        "Non-thread-safe operation invoked on an event loop other "
        "than the current one")
RuntimeError: Non-thread-safe operation invoked on an event loop other than the current one

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/user/code/electrum-fork/electrum/plugin.py", line 184, in load_plugins
    self.load_plugin_by_name(name)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/user/code/electrum-fork/electrum/plugin.py", line 631, in load_plugin_by_name
    raise Exception(f"Error loading {name} plugin: {repr(e)}") from e
Exception: Error loading watchtower plugin: RuntimeError('Non-thread-safe operation invoked on an event loop other than the current one')

```

Author: f321x

electrum/plugins/watchtower/watchtower.py

@@ -56,7 +56,7 @@ def __init__(self, parent, config: 'SimpleConfig', name):
             return
 
         self.watchtower = WatchTower(self.network)
-        asyncio.ensure_future(self.watchtower.start_watching())
+        asyncio.run_coroutine_threadsafe(self.watchtower.start_watching(), self.network.asyncio_loop)
         if watchtower_port := self.config.WATCHTOWER_SERVER_PORT:
             self.server = WatchTowerServer(self.watchtower, self.network, watchtower_port)
             asyncio.run_coroutine_threadsafe(self.network.taskgroup.spawn(self.server.run), self.network.asyncio_loop)