fix: use immutable cache for historical year queries (JhaSourav07#2108)

## Description
Fixes JhaSourav07#2101 

When using the `?year=` parameter to query a past year (e.g.
`?year=2023`), the API was caching responses with a midnight-expiring
TTL — same as live queries. Since historical year data is immutable,
this caused unnecessary CDN re-fetching every 24 hours.

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

## Visual Preview
**Historical year (`?year=2023`):**
cache-control: public, s-maxage=31536000, immutable
**Current year (no year param):**
cache-control: public, s-maxage=40362, stale-while-revalidate=86400

## 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).
- [ ] I have run `npm run test` and all tests pass locally.
- [ ] I have run `npm run test:coverage` and branch coverage is at or
above 70%.
- [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.
- [ ] I have started 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).
- [X] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support.

Author: JhaSourav07

app/api/streak/route.ts

@@ -106,6 +106,8 @@ export async function GET(request: Request) {
       : year
         ? `${year}-12-31T23:59:59Z`
         : undefined;
+    const currentYear = new Date().getUTCFullYear();
+    const isHistoricalYear = !!year && Number(year) < currentYear;
 
     let timezone = 'UTC';
     if (tzParam) {
@@ -224,7 +226,9 @@ export async function GET(request: Request) {
       : getSecondsUntilUTCMidnight();
     const cacheControl = refresh
       ? 'no-cache, no-store, must-revalidate'
-      : `public, s-maxage=${secondsToMidnight}, stale-while-revalidate=86400`;
+      : isHistoricalYear
+        ? 'public, s-maxage=31536000, immutable'
+        : `public, s-maxage=${secondsToMidnight}, stale-while-revalidate=86400`;
 
     return new NextResponse(svg, {
       headers: {