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/main.py

@@ -75,6 +75,7 @@ async def lifespan(app: FastAPI):
         migrate_notification_slot_locale,
         migrate_user_token_version,
         migrate_performance_indexes,
+        migrate_chat_action_to_column,
         migrate_schema_version,
     )
     from .database.snapshot import snapshot_and_prune
@@ -98,6 +99,7 @@ async def lifespan(app: FastAPI):
     await migrate_notification_slot_locale(engine)
     await migrate_user_token_version(engine)
     await migrate_performance_indexes(engine)
+    await migrate_chat_action_to_column(engine)
     await migrate_schema_version(engine)
     from .database.seeds import seed_all
     await seed_all()