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

postgres_test.go

@@ -155,7 +155,7 @@ func migrated(t *testing.T) (*backpack.App, []party.Plugin, *gorm.DB) {
 
 var bmTables = []string{
 	"jz_bm_meetings", "jz_bm_courses", "jz_bm_class_sessions", "jz_bm_deadlines", "jz_bm_readings",
-	"jz_bm_reading_progress", "jz_bm_tags", "jz_bm_materials", "jz_bm_material_tag", "jz_bm_calendar_feeds",
+	"jz_bm_reading_progress", "jz_bm_reading_ownership", "jz_bm_tags", "jz_bm_materials", "jz_bm_material_tag", "jz_bm_calendar_feeds",
 }
 
 func TestMigrateUpAndRollback(t *testing.T) {