feat: add board group models and update related interfaces

backend/app/models/board_group_memory.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID, uuid4
+
+from sqlalchemy import JSON, Column
+from sqlmodel import Field, SQLModel
+
+from app.core.time import utcnow
+
+
+class BoardGroupMemory(SQLModel, table=True):
+    __tablename__ = "board_group_memory"
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    board_group_id: UUID = Field(foreign_key="board_groups.id", index=True)
+    content: str
+    tags: list[str] | None = Field(default=None, sa_column=Column(JSON))
+    is_chat: bool = Field(default=False, index=True)
+    source: str | None = None
+    created_at: datetime = Field(default_factory=utcnow)