feat: add feedback acknowledgement

internal/model/feedback.go

@@ -21,6 +21,10 @@ type Feedback struct {
 	Message      string     `json:"message"`
 	ContactEmail *string    `json:"contact_email,omitempty"`
 	CreatedAt    time.Time  `json:"created_at"`
+	// AcknowledgedAt is set when an admin presses "Acknowledge" on the
+	// /stats page — the team has seen it and (if it needs work) the devs
+	// have started. NULL until then. One-way like ResolvedAt.
+	AcknowledgedAt *time.Time `json:"acknowledged_at,omitempty"`
 	// ResolvedAt is set the moment an admin presses "Mark resolved" on
 	// the /stats page. NULL while open. The action is one-way — the
 	// repo's MarkResolved guards on `resolved_at IS NULL` so a second