feat: per-student owned flag on readings

- new table jz_bm_reading_ownership and its migration
  20261026120000_create_jz_bm_reading_ownership
- PUT and DELETE /_bm/api/v1/readings/{id}/owned, idempotent
- owned_at on every reading in /readings and /courses/{id}
- read semantics, progress routes and counts unchanged

controllers/api/json.go

@@ -168,7 +168,7 @@ func newDeadlinesJSON(rows []models.Deadline) []deadlineJSON {
 	return out
 }
 
-// readingJSON is one reading with the caller's read state.
+// readingJSON is one reading with the caller's read and owned state.
 type readingJSON struct {
 	ID         uint       `json:"id"`
 	CourseID   uint       `json:"course_id"`
@@ -180,6 +180,7 @@ type readingJSON struct {
 	IsRequired bool       `json:"is_required"`
 	SortOrder  int        `json:"sort_order"`
 	ReadAt     *string    `json:"read_at"`
+	OwnedAt    *string    `json:"owned_at"`
 }
 
 // tagJSON is one material tag.