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

frontend/src/lib/i18n/ru.json

@@ -338,6 +338,7 @@
 		"chatName": "Имя",
 		"chatType": "Тип",
 		"chatLang": "Язык",
+		"langOverride": "Переопр.",
 		"cmds": "Команды",
 		"commandsToggle": "Включить/выключить команды для этого чата",
 		"chatId": "ID чата",