refactor: update module docstrings for clarity and consistency

backend/app/schemas/errors.py

@@ -1,12 +1,18 @@
+"""Structured error payload schemas used by API responses."""
+
 from __future__ import annotations
 
 from sqlmodel import Field, SQLModel
 
 
 class BlockedTaskDetail(SQLModel):
+    """Error detail payload listing blocking dependency task identifiers."""
+
     message: str
     blocked_by_task_ids: list[str] = Field(default_factory=list)
 
 
 class BlockedTaskError(SQLModel):
+    """Top-level blocked-task error response envelope."""
+
     detail: BlockedTaskDetail