feat(api): extract difficulty and set from issue labels in catalog reconciliation

apps/api/src/catalog.ts

@@ -4,11 +4,19 @@
  * rows from scratch on every run — re-runs are no-ops. SRS-owned columns
  * (stage, next_review) are NEVER overwritten; defer_until is set once, on
  * first sight of a deferred problem. The Worker never invents catalog rows.
+ *
+ * Difficulty and set come from the `diff:*` / `set:*` LABELS, never from the
+ * issue title — the title carries only the LC number and the problem's name,
+ * 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 { addDays } from "./srs.ts";
 import type { GitHub } from "./github.ts";
 
-const TITLE_RE = /^LC (\d+) · (.+) · (Easy|Medium|Hard) · (core|optional|deferred)$/;
+const TITLE_RE = /^LC (\d+) · (.+)$/;
+
+const DIFFICULTIES = ["easy", "medium", "hard"] as const;
+const SETS = ["core", "optional", "deferred"] as const;
 
 /** Deferred Hards enter the queue from this date, two per day. */
 const DEFER_FROM = "2026-09-28";
@@ -18,6 +26,18 @@ export interface ReconcileReport {
   problems: number;
 }
 
+/** Label names on one issue payload, narrowed with guards rather than cast. */
+function labelNames(value: object): string[] {
+  if (!("labels" in value) || !Array.isArray(value.labels)) return [];
+  const names: string[] = [];
+  for (const label of value.labels) {
+    if (label && typeof label === "object" && "name" in label && typeof label.name === "string") {
+      names.push(label.name);
+    }
+  }
+  return names;
+}
+
 export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<ReconcileReport> {
   interface TopicIssue {
     number: number;
@@ -62,10 +82,20 @@ export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<Reco
       if (!raw || typeof raw !== "object") continue;
       if (!("number" in raw) || typeof raw.number !== "number") continue;
       if (!("title" in raw) || typeof raw.title !== "string") continue;
+      const labels = labelNames(raw);
+      if (!labels.includes("problem")) continue; // non-curriculum sub-issue
       const m = raw.title.match(TITLE_RE);
-      if (!m) continue; // non-curriculum sub-issue
+      if (!m) {
+        console.warn(`catalog: #${raw.number} is a problem but not titled "LC <n> · <name>"`);
+        continue;
+      }
+      const difficulty = DIFFICULTIES.find((d) => labels.includes(`diff:${d}`));
+      const set = SETS.find((s) => labels.includes(`set:${s}`));
+      if (!difficulty || !set) {
+        console.warn(`catalog: #${raw.number} is missing a diff:/set: label — skipped`);
+        continue;
+      }
       const lc = Number(m[1]);
-      const set = m[4]!;
       // Two deferred Hards per day from DEFER_FROM, in catalog walk order —
       // applied only when the row is first created (SRS owns it afterwards).
       const defer = set === "deferred" ? addDays(DEFER_FROM, Math.floor(deferredSeen / 2)) : null;
@@ -78,7 +108,7 @@ export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<Reco
              ON CONFLICT(lc_number) DO UPDATE SET
                issue = ?2, topic_issue = ?3, title = ?4, difficulty = ?5, set_label = ?6`,
           )
-          .bind(lc, raw.number, t.number, m[2]!, m[3]!.toLowerCase(), set, defer),
+          .bind(lc, raw.number, t.number, m[2]!, difficulty, set, defer),
       );
       problems++;
     }