feat(api): enforce REVIEW_CAP, level overflow reviews, and display difficulty in digest

apps/api/src/srs.ts

@@ -165,6 +165,31 @@ export async function dueReviews(db: D1Database, date: string): Promise<ProblemR
   return results;
 }
 
+/** Reviews surfaced per day; the digest levels everything past this forward. */
+export const REVIEW_CAP = 3;
+
+/**
+ * Load-level an overloaded review queue: everything due beyond today's
+ * REVIEW_CAP is pushed to a concrete future date — at most REVIEW_CAP per
+ * day, oldest first — instead of piling up as "due today". Runs every digest
+ * morning, so a future day that grows past the cap (spill plus newly
+ * maturing reviews) is simply re-levelled when it arrives. Idempotent within
+ * a date: after one pass at most REVIEW_CAP problems remain due today, so a
+ * second pass moves nothing.
+ */
+export async function levelReviews(db: D1Database, date: string): Promise<number> {
+  const overflow = (await dueReviews(db, date)).slice(REVIEW_CAP);
+  if (overflow.length === 0) return 0;
+  await db.batch(
+    overflow.map((p, i) =>
+      db
+        .prepare("UPDATE problems SET next_review = ? WHERE lc_number = ?")
+        .bind(addDays(date, 1 + Math.floor(i / REVIEW_CAP)), p.lc_number),
+    ),
+  );
+  return overflow.length;
+}
+
 /**
  * Blind drills: unsolved optional problems from topics ALREADY LEARNED —
  * scheduled in an earlier week (the current week's optional pool is reserved