feat(api): add concept temperature bands and expose in stats

apps/api/src/catalog.ts

@@ -10,14 +10,11 @@
  * so relabelling a problem is enough to move it and titles stay readable.
  * The `problem` label is what marks a sub-issue as curriculum.
  */
-import { workingDay } from "./srs.ts";
+import { DIFFICULTIES, SETS, workingDay } from "./srs.ts";
 import type { GitHub } from "./github.ts";
 
 const TITLE_RE = /^LC (\d+) · (.+)$/;
 
-const DIFFICULTIES = ["easy", "medium", "hard"] as const;
-const SETS = ["core", "optional", "deferred"] as const;
-
 /** Deferred Hards enter the queue from this date, two per working day. */
 const DEFER_FROM = "2026-09-28";