fix(backend): delete migration losers before winner rewrite

backend/store.go

@@ -192,22 +192,23 @@ func migrateAsuraKeys(db *sql.DB) error {
 				winner = i
 			}
 		}
+		// Losers go first: rewriting the winner to the stripped key while a
+		// pre-existing hashless row still holds it is a primary-key collision.
 		for i, r := range g {
 			if i == winner {
-				if r.id == stripped {
-					continue // already stable
-				}
-				if _, err := db.Exec(
-					`UPDATE bookmarks SET key = ?, series_id = ? WHERE key = ?`,
-					"asura:"+stripped, stripped, r.key); err != nil {
-					return fmt.Errorf("rewrite key %q: %w", r.key, err)
-				}
 				continue
 			}
 			if _, err := db.Exec(`DELETE FROM bookmarks WHERE key = ?`, r.key); err != nil {
 				return fmt.Errorf("drop duplicate %q: %w", r.key, err)
 			}
 		}
+		if r := g[winner]; r.id != stripped {
+			if _, err := db.Exec(
+				`UPDATE bookmarks SET key = ?, series_id = ? WHERE key = ?`,
+				"asura:"+stripped, stripped, r.key); err != nil {
+				return fmt.Errorf("rewrite key %q: %w", r.key, err)
+			}
+		}
 	}
 	return nil
 }