feat: add board goal schemas and validation

backend/app/schemas/board_onboarding.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID
+
+from sqlmodel import SQLModel
+
+
+class BoardOnboardingStart(SQLModel):
+    pass
+
+
+class BoardOnboardingAnswer(SQLModel):
+    answer: str
+    other_text: str | None = None
+
+
+class BoardOnboardingConfirm(SQLModel):
+    board_type: str
+    objective: str | None = None
+    success_metrics: dict[str, object] | None = None
+    target_date: datetime | None = None
+
+
+class BoardOnboardingRead(SQLModel):
+    id: UUID
+    board_id: UUID
+    session_key: str
+    status: str
+    messages: list[dict[str, object]] | None = None
+    draft_goal: dict[str, object] | None = None
+    created_at: datetime
+    updated_at: datetime