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

models/reading_ownership.go

@@ -0,0 +1,16 @@
+package models
+
+import "time"
+
+// ReadingOwnership records that a user (a golem15.user users row) owns a
+// copy of a reading. One row per reading and user, kept apart from the read
+// mark in ReadingProgress.
+type ReadingOwnership struct {
+	ID        uint      `gorm:"column:id;primaryKey"`
+	ReadingID uint      `gorm:"column:reading_id"`
+	UserID    uint      `gorm:"column:user_id"`
+	OwnedAt   time.Time `gorm:"column:owned_at"`
+}
+
+// TableName is the plugin table.
+func (ReadingOwnership) TableName() string { return "jz_bm_reading_ownership" }