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/notification_tracker_targets.py

@@ -15,6 +15,7 @@ from ..database.models import (
     NotificationTracker,
     NotificationTrackerTarget,
     ServiceProvider,
+    TargetReceiver,
     TemplateConfig,
     TemplateSlot,
     TrackingConfig,
@@ -206,8 +207,39 @@ async def test_notification_tracker_target(
     if not target:
         raise HTTPException(status_code=404, detail="Target not found")
 
+    # Resolve effective locale from first receiver (explicit locale → TelegramChat lang)
+    effective_locale = locale
+    recv_result = await session.exec(
+        select(TargetReceiver).where(
+            TargetReceiver.target_id == target.id,
+            TargetReceiver.enabled == True,
+        )
+    )
+    first_recv = recv_result.first()
+    if first_recv:
+        recv_locale = getattr(first_recv, 'locale', '') or ''
+        if not recv_locale and target.type == "telegram":
+            # Resolve from TelegramChat language_override/language_code
+            from ..database.models import TelegramBot, TelegramChat
+            chat_id = str(first_recv.config.get("chat_id", ""))
+            bot_id = target.config.get("bot_id")
+            if chat_id and bot_id:
+                chat_row = (await session.exec(
+                    select(TelegramChat).where(
+                        TelegramChat.bot_id == bot_id,
+                        TelegramChat.chat_id == chat_id,
+                    )
+                )).first()
+                if chat_row:
+                    recv_locale = (
+                        getattr(chat_row, 'language_override', '') or
+                        getattr(chat_row, 'language_code', '') or ''
+                    )
+        if recv_locale:
+            effective_locale = recv_locale[:2].lower()
+
     if test_type == "basic":
-        r = await send_test_notification(target, locale=locale)
+        r = await send_test_notification(target, locale=effective_locale)
         return {"target": target.name, **r}
 
     # For periodic/scheduled/memory — fetch real data from provider
@@ -226,7 +258,7 @@ async def test_notification_tracker_target(
                 select(TemplateSlot).where(
                     TemplateSlot.config_id == template_config.id,
                     TemplateSlot.slot_name == slot_name,
-                    TemplateSlot.locale == "en",
+                    TemplateSlot.locale == effective_locale,
                 )
             )
             slot = slot_result.first()

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(),
     }

packages/server/src/notify_bridge_server/api/targets.py

@@ -107,7 +107,7 @@ async def list_targets(
                     chat = chat_result.first()
                     if chat:
                         chat_names[f"{bot_id}_{chat_id}"] = chat.title or chat.username or ""
-                        lang = getattr(chat, 'language_code', '') or ''
+                        lang = getattr(chat, 'language_override', '') or getattr(chat, 'language_code', '') or ''
                         if lang:
                             chat_languages[f"{bot_id}_{chat_id}"] = lang
 
@@ -278,6 +278,7 @@ def _target_response(
                 "name": r.name,
                 "config": dict(r.config),
                 "receiver_key": r.receiver_key,
+                "locale": getattr(r, 'locale', '') or '',
                 "enabled": r.enabled,
             }
             for r in recv_list

packages/server/src/notify_bridge_server/api/telegram_bots.py

@@ -15,7 +15,7 @@ from ..auth.dependencies import get_current_user
 from ..commands.handler import register_commands_with_telegram
 from ..commands.webhook import register_webhook, unregister_webhook
 from ..database.engine import get_session
-from ..database.models import AppSetting, TelegramBot, TelegramChat, User
+from ..database.models import AppSetting, NotificationTarget, TargetReceiver, TelegramBot, TelegramChat, User
 from ..services.notifier import _get_test_message
 from ..services.telegram_poller import schedule_bot_polling, unschedule_bot_polling
 from .app_settings import get_setting
@@ -297,7 +297,7 @@ async def test_chat(
 
 
 class ChatUpdate(BaseModel):
-    language_code: str | None = None
+    language_override: str | None = None
     title: str | None = None
     commands_enabled: bool | None = None
 
@@ -389,6 +389,7 @@ def _chat_response(c: TelegramChat) -> dict:
         "type": c.chat_type,
         "username": c.username,
         "language_code": getattr(c, 'language_code', '') or '',
+        "language_override": getattr(c, 'language_override', '') or '',
         "commands_enabled": getattr(c, 'commands_enabled', False),
         "discovered_at": c.discovered_at.isoformat(),
     }