feat(commands): keep chat-action hint alive during slow command fetches

Slow bot commands (/latest, /random, /favorites, /memory, /search,
/find, /person, /place, /summary) spend most of their wall time
fetching assets from the service provider, not uploading to Telegram.
Telegram chat actions expire after ~5s, so the previous one-shot hint
vanished long before media arrived — users saw nothing happening.

- TelegramClient.start_chat_action_keepalive: promoted from private
  helper to public API, posts the action every 4s until cancelled.
- telegram_send.telegram_chat_action: async context manager that
  starts the keep-alive task on enter and cancels + awaits it on
  exit. A None action makes it a no-op so callers don't branch.
- classify_command_chat_action: maps command name to the right
  Telegram action (upload_photo for media-returning commands, typing
  for /summary, None for fast DB-only commands like /status /events).
- webhook.py + telegram_poller.py: wrap handle_command in the context
  manager so the hint persists through the whole fetch+upload window
  in both webhook and long-poll modes.

packages/core/src/notify_bridge_core/notifications/telegram/client.py

@@ -229,7 +229,7 @@ class TelegramClient:
 
         typing_task = None
         if chat_action:
-            typing_task = self._start_typing_indicator(chat_id, chat_action)
+            typing_task = self.start_chat_action_keepalive(chat_id, chat_action)
 
         try:
             if len(assets) == 1 and assets[0].get("type") == "photo":
@@ -340,7 +340,13 @@ class TelegramClient:
         except aiohttp.ClientError:
             return False
 
-    def _start_typing_indicator(self, chat_id: str, action: str = "typing") -> asyncio.Task:
+    def start_chat_action_keepalive(self, chat_id: str, action: str = "typing") -> asyncio.Task:
+        """Repeatedly post ``action`` every 4s until the returned task is cancelled.
+
+        Telegram chat actions expire after ~5s, so callers that want the hint
+        to persist through longer work (fetching assets, multi-chunk uploads)
+        need a keep-alive. Cancel the task in a ``finally`` to stop it.
+        """
         async def action_loop() -> None:
             try:
                 while True: