feat: проверка существования канала при привязке

src/usecase/__init__.py

@@ -279,11 +279,22 @@ class JWTEncoder(typing.Protocol):
     def encode_access_token(self, user_id: UUID, telegram_id: int, username: str | None = None) -> str: ...
 
 
+class FetchChannelResponse(typing.Protocol):
+    telegram_id: int
+    username: str
+    title: str
+
+
+class ParserClient(typing.Protocol):
+    async def fetch_telegram_channel(self, username: str) -> FetchChannelResponse | None: ...
+
+
 @dataclass
 class Usecase:
     database: Database
     telegram_bot: TelegramBotWriter
     jwt_encoder: JWTEncoder
+    parser_client: ParserClient
 
     async def ensure_workspace_access(self, workspace_id: UUID, user_id: UUID) -> domain.Workspace:
         workspace = await self.database.get_workspace_for_user(workspace_id, user_id)