feat: add open questions with a pending grading state

- migration: open question type, rubric, max_length, quiz grader and
  the grading and review columns of attempt answers
- open answers are checked, stored as pending and never scored on
  answer; a changed text resets the grading state
- student JSON gains text, grading_status, grader_feedback,
  review_comment, max_length, needs_review and open_answers; the
  rubric is never serialized

models/attempt_answer.go

@@ -8,8 +8,31 @@ import (
 	"git.golem15.com/golem15/summercms/modules/lagoon"
 )
 
+// Grading states of an open answer (AttemptAnswer.GradingStatus).
+const (
+	// GradingPending waits for a grader or for the owner.
+	GradingPending = "pending"
+	// GradingGraded holds a grader's score the owner has not confirmed.
+	GradingGraded = "graded"
+	// GradingFailed is an answer the grader could not score; the owner
+	// grades it.
+	GradingFailed = "failed"
+	// GradingReviewed is final: the owner confirmed or set the score.
+	GradingReviewed = "reviewed"
+)
+
+// GradingStatuses lists the valid values of AttemptAnswer.GradingStatus.
+var GradingStatuses = []string{GradingPending, GradingGraded, GradingFailed, GradingReviewed}
+
 // AttemptAnswer is the user's answer to one question of an attempt. There is
 // one row per attempt and question; answering again overwrites it.
+//
+// TextAnswer and the grading fields belong to open questions only:
+// GradingStatus is NULL for every other row. GradingSeq grows with every new
+// text, so a grading job for an older text is ignored; GradingAttempts counts
+// the grader runs for the current text. The review fields record the owner's
+// decision: ReviewedByUserID for a frontend user (MCP), ReviewedByAdminID for
+// a backend administrator.
 type AttemptAnswer struct {
 	ID             uint                    `gorm:"column:id;primaryKey"`
 	AttemptID      uint                    `gorm:"column:attempt_id"`
@@ -20,6 +43,20 @@ type AttemptAnswer struct {
 	Points         float64                 `gorm:"column:points"`
 	CreatedAt      time.Time               `gorm:"column:created_at"`
 	UpdatedAt      time.Time               `gorm:"column:updated_at"`
+
+	TextAnswer        *string    `gorm:"column:text_answer"`
+	GradingStatus     *string    `gorm:"column:grading_status"`
+	GradingSeq        int        `gorm:"column:grading_seq"`
+	GradingAttempts   int        `gorm:"column:grading_attempts"`
+	Grader            *string    `gorm:"column:grader"`
+	GraderPoints      *float64   `gorm:"column:grader_points"`
+	GraderFeedback    *string    `gorm:"column:grader_feedback"`
+	GradingError      *string    `gorm:"column:grading_error"`
+	GradedAt          *time.Time `gorm:"column:graded_at"`
+	ReviewComment     *string    `gorm:"column:review_comment"`
+	ReviewedAt        *time.Time `gorm:"column:reviewed_at"`
+	ReviewedByUserID  *uint      `gorm:"column:reviewed_by_user_id"`
+	ReviewedByAdminID *uint      `gorm:"column:reviewed_by_admin_id"`
 }
 
 // TableName is the plugin-prefixed table.