feat: add config read-modify-write update and harden file watcher

Config.update_config() applies submitted options to the main
configuration file via configparser read-modify-write: unknown
options are preserved, null removes an option, the merged view
of all files is validated as ServerConfig before anything is
written (a bad file would kill the FileWatcher polling loop),
and the write is atomic (.tmp + os.replace, mode 0600). Options
whose effective value is owned by a later configuration file
raise ConfigConflictError instead of writing a no-op. The
reload logic is factored into reload_and_notify() and the file
watcher callback is exception-guarded so polling never dies.

tests/utils/test_file_watcher.py

@@ -66,3 +66,20 @@ async def test_file_watcher_list(tmpdir, strategy):
     file2.write("b")
     await asyncio.sleep(0.5)
     callback.assert_called_with(str(file2))
+
+
+@pytest.mark.asyncio
+async def test_file_watcher_callback_exception_does_not_stop_polling(tmpdir):
+
+    file = tmpdir / "test"
+    file.write("a")
+    callback = MagicMock(side_effect=ValueError("callback error"))
+    FileWatcher(file, callback, delay=0.1)
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 0
+    file.write("b")
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 1  # raised, but must not kill the polling loop
+    file.write("c")
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 2  # polling survived the exception