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/api/target_receivers.py

@@ -21,12 +21,14 @@ router = APIRouter(prefix="/api/targets/{target_id}/receivers", tags=["target-re
 class ReceiverCreate(BaseModel):
     name: str = ""
     config: dict[str, Any] = {}
+    locale: str = ""
     enabled: bool = True
 
 
 class ReceiverUpdate(BaseModel):
     name: str | None = None
     config: dict[str, Any] | None = None
+    locale: str | None = None
     enabled: bool | None = None
 
 
@@ -85,6 +87,7 @@ async def create_receiver(
         name=body.name,
         config=body.config,
         receiver_key=key,
+        locale=body.locale,
         enabled=body.enabled,
     )
     session.add(receiver)
@@ -133,7 +136,8 @@ async def test_receiver(
         raise HTTPException(status_code=404, detail="Receiver not found")
 
     from ..services.notifier import _get_test_message
-    message = _get_test_message(locale, target.type)
+    effective_locale = getattr(receiver, 'locale', '') or locale
+    message = _get_test_message(effective_locale, target.type)
     return await send_to_receiver(target, dict(receiver.config), message)
 
 
@@ -159,6 +163,7 @@ def _response(r: TargetReceiver) -> dict:
         "name": r.name,
         "config": dict(r.config),
         "receiver_key": r.receiver_key,
+        "locale": getattr(r, 'locale', '') or '',
         "enabled": r.enabled,
         "created_at": r.created_at.isoformat(),
     }