feat: add validation for minimum length on various fields and update type definitions

backend/app/schemas/common.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+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.
+NonEmptyStr = Annotated[str, StringConstraints(strip_whitespace=True, min_length=1)]
+
+
+class OkResponse(SQLModel):
+    ok: bool = True