feat(telegram,core): master topic instead of general in private chats

src/beaver_gateway/core/conversations.py

@@ -345,6 +345,28 @@ class Conversations:
             )
             return result.first()
 
+    async def last_binding(
+        self, *, frontend: str, kind: str
+    ) -> ConversationBinding | None:
+        """The window ``frontend`` last used for a conversation of ``kind``.
+
+        A frontend whose window for the master outlives the master itself
+        (the Telegram General topic) finds it here after a rotation.
+        """
+        async with self._db.session() as session:
+            result = await session.exec(
+                select(ConversationBinding)
+                .join(
+                    Conversation,
+                    col(Conversation.id) == col(ConversationBinding.conversation_id),
+                )
+                .where(
+                    ConversationBinding.frontend == frontend, Conversation.kind == kind
+                )
+                .order_by(col(ConversationBinding.id).desc())
+            )
+            return result.first()
+
     async def set_flags(
         self, conv: Conversation, flags: dict[str, Any]
     ) -> Conversation: