feat(backend): add publish readiness contract and guarded publish flow

add backend readiness contract for publish prechecks

guard publish flow with explicit validation and version-aware checks
make publish behavior more predictable for clients and safer against stale state

backend/app/services/api_errors.py

@@ -3,40 +3,29 @@ from __future__ import annotations
 from fastapi import HTTPException, status
 
 
-def build_error_detail(
-    *,
-    code: str,
-    message: str,
-    details: dict | None = None,
-) -> dict:
-    payload = {
+def raise_conflict(*, code: str, message: str, details: dict | None = None) -> None:
+    payload: dict = {
         "code": code,
         "message": message,
     }
     if details:
         payload["details"] = details
-    return payload
 
-
-def raise_conflict(
-    *,
-    code: str,
-    message: str,
-    details: dict | None = None,
-) -> None:
     raise HTTPException(
         status_code=status.HTTP_409_CONFLICT,
-        detail=build_error_detail(code=code, message=message, details=details),
+        detail=payload,
     )
 
 
-def raise_unprocessable(
-    *,
-    code: str,
-    message: str,
-    details: dict | None = None,
-) -> None:
+def raise_unprocessable(*, code: str, message: str, details: dict | None = None) -> None:
+    payload: dict = {
+        "code": code,
+        "message": message,
+    }
+    if details:
+        payload["details"] = details
+
     raise HTTPException(
         status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-        detail=build_error_detail(code=code, message=message, details=details),
+        detail=payload,
     )