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/database/migrations.py

@@ -1391,6 +1391,40 @@ async def migrate_performance_indexes(engine: AsyncEngine) -> None:
                 )
 
 
+async def migrate_chat_action_to_column(engine: AsyncEngine) -> None:
+    """Move ``chat_action`` from ``config`` JSON to the dedicated column.
+
+    Earlier versions of the frontend stored ``chat_action`` inside
+    ``notification_target.config``; the dedicated ``chat_action`` column
+    was rarely set or held a stale default. The dispatcher's resolver
+    overrode the config value with the (stale) column, so a user's UI
+    choice silently had no effect on outgoing chat actions.
+
+    This backfill takes the config value as authoritative (it's what the
+    UI was writing) and copies it to the column, then strips it from
+    config so the column becomes the single source of truth. Idempotent:
+    a second run finds nothing to migrate.
+    """
+    async with engine.begin() as conn:
+        if not await _has_table(conn, "notification_target"):
+            return
+        if not await _has_column(conn, "notification_target", "chat_action"):
+            return
+        # Copy config["chat_action"] → column where present.
+        await conn.execute(text(
+            "UPDATE notification_target "
+            "SET chat_action = json_extract(config, '$.chat_action') "
+            "WHERE json_extract(config, '$.chat_action') IS NOT NULL"
+        ))
+        # Strip the legacy key so the column is unambiguous going forward.
+        await conn.execute(text(
+            "UPDATE notification_target "
+            "SET config = json_remove(config, '$.chat_action') "
+            "WHERE json_extract(config, '$.chat_action') IS NOT NULL"
+        ))
+        logger.info("Migrated chat_action from config JSON to column where present")
+
+
 # ---------------------------------------------------------------------------
 # Schema version tracking — lightweight alternative to Alembic while the
 # hand-rolled idempotent migrations remain the source of truth. Gives