feat: Receiver OOP hierarchy with per-receiver locale resolution

- Introduce Receiver base class + typed subclasses (TelegramReceiver,
  WebhookReceiver, EmailReceiver, etc.) in core/notifications/receiver.py
- Dispatcher uses typed Receiver objects instead of raw dicts, with
  per-receiver locale-aware template rendering
- load_link_data resolves locale from TelegramChat.language_override at
  load time: TargetReceiver.locale || chat.language_override || chat.language_code
- Add language_override field to TelegramChat (separate from auto-detected
  language_code), with per-chat commands toggle and command dispatch using
  override language
- Add locale field to TargetReceiver for explicit per-receiver overrides

packages/server/src/notify_bridge_server/commands/webhook.py

@@ -86,8 +86,9 @@ async def telegram_webhook(
         )).first()
         if not chat_row or not chat_row.commands_enabled:
             return {"ok": True, "skipped": "commands_disabled"}
+        effective_lang = chat_row.language_override or msg_language
         message_id = message.get("message_id")
-        cmd_response = await handle_command(bot, chat_id, text, language_code=msg_language)
+        cmd_response = await handle_command(bot, chat_id, text, language_code=effective_lang)
         if cmd_response is not None:
             if isinstance(cmd_response, list):
                 await send_media_group(bot.token, chat_id, cmd_response, reply_to_message_id=message_id)