refactor: update module docstrings for clarity and consistency

backend/app/models/board_onboarding.py

@@ -1,6 +1,8 @@
+"""Board onboarding session model for guided setup state."""
+
 from __future__ import annotations
 
-from datetime import datetime
+from datetime import datetime  # noqa: TCH003
 from uuid import UUID, uuid4
 
 from sqlalchemy import JSON, Column
@@ -11,13 +13,18 @@ from app.models.base import QueryModel
 
 
 class BoardOnboardingSession(QueryModel, table=True):
+    """Persisted onboarding conversation and draft goal data for a board."""
+
     __tablename__ = "board_onboarding_sessions"
 
     id: UUID = Field(default_factory=uuid4, primary_key=True)
     board_id: UUID = Field(foreign_key="boards.id", index=True)
     session_key: str
     status: str = Field(default="active", index=True)
-    messages: list[dict[str, object]] | None = Field(default=None, sa_column=Column(JSON))
+    messages: list[dict[str, object]] | None = Field(
+        default=None,
+        sa_column=Column(JSON),
+    )
     draft_goal: dict[str, object] | None = Field(default=None, sa_column=Column(JSON))
     created_at: datetime = Field(default_factory=utcnow)
     updated_at: datetime = Field(default_factory=utcnow)