fix(cache): make DistributedCache.incr() fail closed on Redis errors

[taniy8]

Description

Fixes #6138

Problem

DistributedCache.incr() in lib/cache.ts is the core primitive behind
every rate limiter in the app (lib/rate-limit.ts calls cache.incr() for
all per-IP rate limit checks). When the Redis call failed (network error,
timeout, Upstash outage), the catch block silently fell back to the LOCAL
in-memory TTLCache as if it were the authoritative distributed counter.

Serverless instances don't share memory, so each instance would maintain
its own independent counter starting from 0 during any Redis blip. With
N concurrent instances, an attacker effectively gets N times the intended
rate limit defeating rate limiting across every protected endpoint
(/api/achievements, /api/compare, /api/og, /api/user-details, /api/ci-analytics,
/api/notify, /api/webhook, etc.) for the duration of the Redis instability.

Fix

Changed incr() to fail closed on Redis errors instead of failing open.
On failure, it now returns Number.MAX_SAFE_INTEGER, which
RateLimiter.check() already correctly treats as "limit exceeded"
(count > this.limit), no changes needed elsewhere. During a Redis
outage, requests are now blocked rather than silently allowed through
unlimited, which is the safer default for a security control.

Added a test simulating two separate DistributedCache instances
(representing two serverless instances) both calling incr() on the
same key while Redis is down, verifying both independently fail closed
rather than each starting an unsynced local counter.

Pillar

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

Visual Preview

N/A

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• 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.
• 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.

[vercel]

@taniy8 is attempting to deploy a commit to the jhasourav07's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3697.00 KB	3697.00 KB	0 B
Total CSS	296.58 KB	296.58 KB	0 B