Backend API updated to indicate nullable fields

internal/httpx/problem.go

@@ -8,12 +8,12 @@ import (
 
 // Problem is an RFC 9457 problem details body.
 type Problem struct {
-	Type      string `json:"type,omitempty"`
+	Type      string `json:"type,omitempty"      validate:"optional"`
 	Status    int    `json:"status"`
 	Title     string `json:"title"`
-	Detail    string `json:"detail,omitempty"`
-	Instance  string `json:"instance,omitempty"`
-	RequestID string `json:"requestId,omitempty"`
+	Detail    string `json:"detail,omitempty"    validate:"optional"`
+	Instance  string `json:"instance,omitempty"  validate:"optional"`
+	RequestID string `json:"requestId,omitempty" validate:"optional"`
 }
 
 // WriteProblem writes a problem+json error response.