Align DeliveryResult tags and make the index table test-local

Re-align the json tag column in DeliveryResult after the wider
DeliveryID gorm tag, and move the test's index table out of package
scope into the test function.

Model: opus-5-5

internal/database/model_delivery_result.go

@@ -5,10 +5,10 @@ type DeliveryResult struct {
 	BaseModel
 
 	DeliveryID   string `gorm:"type:uuid;not null;index" json:"deliveryId"`
-	AttemptNum   int    `gorm:"not null"             json:"attemptNum"`
+	AttemptNum   int    `gorm:"not null"                 json:"attemptNum"`
 	Success      bool   `json:"success"`
 	StatusCode   int    `json:"statusCode,omitempty"`
-	ResponseBody string `gorm:"type:text"            json:"responseBody,omitempty"`
+	ResponseBody string `gorm:"type:text"                json:"responseBody,omitempty"`
 	Error        string `json:"error,omitempty"`
 	Duration     int64  `json:"durationMs"` // Duration in milliseconds