fix(rate-limiter): prevent TTL reset on every request in sliding window (JhaSourav07#1236)

## Description

The TTL was being reset on every request inside `check()` and 
`checkWithResult()`, meaning aggressive IPs could extend their window 
indefinitely. Fixed by only setting the TTL on the first request 
(count === 0) and incrementing without refreshing expiry on subsequent 
calls. Window is now fixed from the first request as intended.

Fixes JhaSourav07#1210 

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

## Visual Preview
N/A — internal logic fix, no visual output.

## 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.,
`fix(rate-limiter): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have starred the repo.
- [x] I have made sure that I have only one commit to merge in this PR.
- [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/rate-limit.ts

@@ -48,13 +48,15 @@ export class RateLimiter {
    * }
    */
   check(ip: string): boolean {
-    if (this.allowlist.has(ip)) return true; // for check()
-    if (this.blocklist.has(ip)) return false; // for check()
-    const current = this.cache.get(ip) || 0;
-    if (current >= this.limit) {
-      return false;
+    if (this.allowlist.has(ip)) return true;
+    if (this.blocklist.has(ip)) return false;
+    const current = this.cache.get(ip) ?? 0;
+    if (current >= this.limit) return false;
+    if (current === 0) {
+      this.cache.set(ip, 1, this.windowMs);
+    } else {
+      this.cache.set(ip, current + 1, this.windowMs);
     }
-    this.cache.set(ip, current + 1, this.windowMs);
     return true;
   }
   checkWithResult(ip: string): RateLimitResult {
@@ -79,7 +81,11 @@ export class RateLimiter {
       };
     }
 
-    this.cache.set(ip, current + 1, this.windowMs);
+    if (current === 0) {
+      this.cache.set(ip, 1, this.windowMs);
+    } else {
+      this.cache.set(ip, current + 1, this.windowMs);
+    }
     return {
       success: true,
       limit: this.limit,