feat: add board group models and update related interfaces

backend/app/schemas/board_group_memory.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID
+
+from sqlmodel import SQLModel
+
+from app.schemas.common import NonEmptyStr
+
+
+class BoardGroupMemoryCreate(SQLModel):
+    # For writes, reject blank/whitespace-only content.
+    content: NonEmptyStr
+    tags: list[str] | None = None
+    source: str | None = None
+
+
+class BoardGroupMemoryRead(SQLModel):
+    id: UUID
+    board_group_id: UUID
+    # For reads, allow legacy rows that may have empty content (avoid response validation 500s).
+    content: str
+    tags: list[str] | None = None
+    source: str | None = None
+    is_chat: bool = False
+    created_at: datetime