Stage 4: alert acknowledgement and comments

- Migration 004: acknowledged_by/acknowledged_at columns on alerts,
  alert_comments table (FK cascade on delete)
- POST /api/alerts/{id}/acknowledge — stamps authed user + timestamp,
  returns updated alert with acknowledged_by username
- DELETE /api/alerts/{id}/acknowledge — clears ack (204)
- GET /api/alerts/{id}/comments — list in chronological order
- POST /api/alerts/{id}/comments — add comment (returns 201)
- DELETE /api/alerts/{id}/comments/{commentID} — own comments only (204)
- All alert queries now LEFT JOIN users for ack username

internal/models/comment.go

@@ -0,0 +1,12 @@
+package models
+
+import "time"
+
+type Comment struct {
+	ID        int64     `json:"id"`
+	AlertID   int64     `json:"alert_id"`
+	UserID    int64     `json:"user_id"`
+	Username  string    `json:"username"`
+	Content   string    `json:"content"`
+	CreatedAt time.Time `json:"created_at"`
+}