refactor(api): unify typed error contract across draft pricing and publish flows

standardize typed error responses across draft, pricing and publish endpoints

reduce contract drift between related flows
keep client-side handling more predictable and consistent

backend/app/services/api_errors.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from fastapi import HTTPException, status
+
+
+def raise_conflict(*, code: str, message: str, **extra) -> None:
+    detail = {
+        "code": code,
+        "message": message,
+        **extra,
+    }
+    raise HTTPException(
+        status_code=status.HTTP_409_CONFLICT,
+        detail=detail,
+    )
+
+
+def raise_unprocessable(*, code: str, message: str, **extra) -> None:
+    detail = {
+        "code": code,
+        "message": message,
+        **extra,
+    }
+    raise HTTPException(
+        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+        detail=detail,
+    )
+
+
+def raise_not_found(*, code: str, message: str, **extra) -> None:
+    detail = {
+        "code": code,
+        "message": message,
+        **extra,
+    }
+    raise HTTPException(
+        status_code=status.HTTP_404_NOT_FOUND,
+        detail=detail,
+    )