fix(telegram): respect chat_action UI choice, drop phantom indicator

chat_action was stored in two places — the model column and config JSON —
and dispatch_helpers unconditionally overrode the config value with the
column. The frontend only ever wrote the JSON path, so the UI choice
silently had no effect on outgoing chat actions.

Make the column the single source of truth: frontend sends chat_action
top-level, dispatch_helpers reads from the column, and a one-time
backfill migrates existing config values to the column and strips the
legacy key.

Also fix a long-standing race where the keepalive's bare sleep(4) +
finally cancel could fire one last sendChatAction after the response
already arrived, leaving a phantom indicator for ~5s. Replace with a
stop event + wait_for so callers can signal stop cleanly via the new
stop_keepalive helper.

packages/server/src/notify_bridge_server/services/dispatch_helpers.py

@@ -326,7 +326,11 @@ async def _resolve_target(
         receivers.append(build_receiver(target.type, dict(r.config), locale))
 
     target_config = dict(target.config)
-    # Inject chat_action for Telegram targets
+    # chat_action lives on the model column — single source of truth.
+    # Strip any legacy/stale value from config so an old config-stored value
+    # can't shadow the user's UI choice. When the column is unset, leave the
+    # key absent so the dispatcher's "typing" fallback applies.
+    target_config.pop("chat_action", None)
     if hasattr(target, 'chat_action') and target.chat_action:
         target_config["chat_action"] = target.chat_action
     # Inject bot credentials for bot-backed target types