fix: invalidate leaderboard cache after visibility settings change

[Ridanshi]

Closes #1779

Problem

The leaderboard is cached at two levels:

Layer	Key	TTL
Module-level in-process variable (_memoryCache)	n/a	1 hour
Shared Redis/Upstash entry	leaderboard:v1	6 hours

PATCH /api/user/settings persists is_public and leaderboard_opt_in changes to the database but does not invalidate either cache layer. A user who opts out of the leaderboard therefore remains visible to other users for up to one hour; a user who opts in remains invisible for the same duration.

Reproduction: change either setting in the dashboard and immediately visit the leaderboard — the old state persists until cache expiry.

Fix

src/lib/metrics-cache.ts — new cacheDelete

export async function cacheDelete(key: string): Promise<void> {
  memoryCache.delete(key);
  const redis = getRedisClient();
  if (!redis) return;
  try { await redis.del(key); } catch { /* best-effort */ }
}

Removes a single key from both the in-process memory Map and Redis/Upstash. Complements the existing invalidateUserMetricsCache (which scans a per-user prefix) for the case where one shared key must be evicted.

src/lib/leaderboard.ts — new clearLeaderboardCache

export async function clearLeaderboardCache(): Promise<void> {
  _memoryCache = null;                        // in-process module-level cache
  await cacheDelete(LEADERBOARD_CACHE_KEY);   // metrics memory + Redis/Upstash
}

Evicts every cache layer so the next leaderboard request must rebuild from the database, reflecting updated preferences immediately.

src/app/api/user/settings/route.ts — call invalidation after relevant PATCHes

const leaderboardEligibilityChanged =
  "is_public" in updates || "leaderboard_opt_in" in updates;

if (leaderboardEligibilityChanged) {
  try {
    await clearLeaderboardCache();
  } catch {
    // best-effort — must not fail the settings response
    console.error("[settings] Failed to invalidate leaderboard cache after visibility change");
  }
}

The check runs only after a successful DB write. It is scoped to the two fields that control leaderboard eligibility; unrelated changes (bio, timezone, etc.) do not trigger a cache bust.

Cache invalidation flow

User toggles is_public / leaderboard_opt_in
  → PATCH /api/user/settings
      → DB write succeeds
      → clearLeaderboardCache()
          → _memoryCache = null                (same process)
          → cacheDelete("leaderboard:v1")
              → memoryCache.delete(key)        (in-process metrics Map)
              → redis.del("leaderboard:v1")    (Redis/Upstash)
      → 200 OK
Next leaderboard request: all cache layers miss → buildLeaderboard() → fresh DB query

Tests — test/leaderboard-cache-invalidation.test.ts (11 new tests)

Category	Tests
Cache cleared for is_public	→ false (regression #1779), → true
Cache cleared for leaderboard_opt_in	→ false (regression #1779), → true
Both change simultaneously	Cleared exactly once
Irrelevant-field changes	bio, timezone, weekly_digest_opt_in — cache NOT cleared
Error resilience	clearLeaderboardCache throws → PATCH still returns 200
Auth guards	401 → no cache call; 404 → no cache call

All 11 pass. The one pre-existing failure in test/dateUtils.test.ts is a timezone boundary issue unrelated to this change.

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[github-actions]

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