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/database/models.py

@@ -95,7 +95,8 @@ class TelegramChat(SQLModel, table=True):
     title: str = Field(default="")
     chat_type: str = Field(default="private")
     username: str = Field(default="")
-    language_code: str = Field(default="")
+    language_code: str = Field(default="")  # auto-detected from Telegram
+    language_override: str = Field(default="")  # manual override set by user
     commands_enabled: bool = Field(default=False)
     discovered_at: datetime = Field(default_factory=_utcnow)
 
@@ -279,6 +280,7 @@ class TargetReceiver(SQLModel, table=True):
     name: str = Field(default="")
     config: dict[str, Any] = Field(default_factory=dict, sa_column=Column(JSON))
     receiver_key: str = Field(default="")  # dedup key (e.g. chat_id, url, email)
+    locale: str = Field(default="")  # e.g. "en", "ru"; empty = use target default
     enabled: bool = Field(default=True)
     created_at: datetime = Field(default_factory=_utcnow)