chore: add comments back to Challenge struct.

See #1284
and https://github.com/TecharoHQ/anubis/pull/1284#issuecomment-3784096905

lib/challenge/challenge.go

@@ -4,12 +4,12 @@ import "time"
 
 // Challenge is the metadata about a single challenge issuance.
 type Challenge struct {
-	IssuedAt       time.Time         `json:"issuedAt"`
-	Metadata       map[string]string `json:"metadata"`
-	ID             string            `json:"id"`
-	Method         string            `json:"method"`
-	RandomData     string            `json:"randomData"`
-	PolicyRuleHash string            `json:"policyRuleHash,omitempty"`
-	Difficulty     int               `json:"difficulty,omitempty"`
-	Spent          bool              `json:"spent"`
+	IssuedAt       time.Time         `json:"issuedAt"`                 // When the challenge was issued
+	Metadata       map[string]string `json:"metadata"`                 // Challenge metadata such as IP address and user agent
+	ID             string            `json:"id"`                       // UUID identifying the challenge
+	Method         string            `json:"method"`                   // Challenge method
+	RandomData     string            `json:"randomData"`               // The random data the client processes
+	PolicyRuleHash string            `json:"policyRuleHash,omitempty"` // Hash of the policy rule that issued this challenge
+	Difficulty     int               `json:"difficulty,omitempty"`     // Difficulty that was in effect when issued
+	Spent          bool              `json:"spent"`                    // Has the challenge already been solved?
 }