refactor: update module docstrings for clarity and consistency

backend/app/schemas/common.py

@@ -1,3 +1,5 @@
+"""Common reusable schema primitives and simple API response envelopes."""
+
 from __future__ import annotations
 
 from typing import Annotated
@@ -5,9 +7,12 @@ from typing import Annotated
 from pydantic import StringConstraints
 from sqlmodel import SQLModel
 
-# Reusable string type for request payloads where blank/whitespace-only values are invalid.
+# Reusable string type for request payloads where blank/whitespace-only values
+# are invalid.
 NonEmptyStr = Annotated[str, StringConstraints(strip_whitespace=True, min_length=1)]
 
 
 class OkResponse(SQLModel):
+    """Standard success response payload."""
+
     ok: bool = True