refactor: update module docstrings for clarity and consistency

backend/app/schemas/activity_events.py

@@ -1,12 +1,16 @@
+"""Response schemas for activity events and task-comment feed items."""
+
 from __future__ import annotations
 
-from datetime import datetime
-from uuid import UUID
+from datetime import datetime  # noqa: TCH003
+from uuid import UUID  # noqa: TCH003
 
 from sqlmodel import SQLModel
 
 
 class ActivityEventRead(SQLModel):
+    """Serialized activity event payload returned by activity endpoints."""
+
     id: UUID
     event_type: str
     message: str | None
@@ -16,6 +20,8 @@ class ActivityEventRead(SQLModel):
 
 
 class ActivityTaskCommentFeedItemRead(SQLModel):
+    """Denormalized task-comment feed item enriched with task and board fields."""
+
     id: UUID
     created_at: datetime
     message: str | None