refactor(cache): add size() method to TTLCache and expose active cache metrics (JhaSourav07#594)

## Description

Fixes JhaSourav07#301

Adds a `size(): number` method to the TTLCache implementation in
`lib/cache.ts` to allow observing the number of active (non-expired)
cache entries. This improves debugging and enables cache metrics without
exposing internal state.

## Pillar

- [ ]  Pillar 1 — New Theme Design
- [ ]  Pillar 2 — Geometric SVG Improvement
- [x] Pillar 3 — Timezone Logic Optimization
- [x] Other (Bug fix, refactoring, docs)

## Visual Preview

(Not applicable — no UI/SVG changes)

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that I have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support.

Author: JhaSourav07

lib/cache.test.ts

@@ -96,6 +96,44 @@ describe('TTLCache', () => {
     });
   });
 
+  describe('size()', () => {
+    it('returns 0 for an empty cache', () => {
+      const cache = new TTLCache<number>();
+      expect(cache.size()).toBe(0);
+      cache.destroy();
+    });
+
+    it('counts only entries before expiry', () => {
+      vi.useFakeTimers();
+      const cache = new TTLCache<number>();
+      cache.set('a', 1, 10_000);
+      cache.set('b', 2, 20_000);
+      expect(cache.size()).toBe(2);
+
+      vi.advanceTimersByTime(15_000);
+      expect(cache.size()).toBe(1);
+      cache.destroy();
+    });
+
+    it('returns 0 when all entries have expired (after TTL expiry)', () => {
+      vi.useFakeTimers();
+      const cache = new TTLCache<number>();
+      cache.set('a', 1, 10_000);
+      vi.advanceTimersByTime(15_000);
+      expect(cache.size()).toBe(0);
+      cache.destroy();
+    });
+
+    it('returns 0 after clear() is called', () => {
+      const cache = new TTLCache<number>();
+      cache.set('a', 1, 10_000);
+      expect(cache.size()).toBe(1);
+      cache.clear();
+      expect(cache.size()).toBe(0);
+      cache.destroy();
+    });
+  });
+
   describe('destroy()', () => {
     it('clears the store and stops the interval', () => {
       vi.useFakeTimers();

lib/cache.ts

@@ -120,11 +120,11 @@ export class TTLCache<T> {
     this.store.clear();
   }
 
-  /**
-   * Stops the cleanup interval and clears the cache.
-   *
-   * @returns void
-   */
+  size(): number {
+    this.sweep();
+    return this.store.size;
+  }
+
   destroy(): void {
     if (this.cleanupInterval) {
       clearInterval(this.cleanupInterval);