chore: review and improve MongoDB connection handling for serverless …

[TanCodeX]

Description

Fixes #1901

This PR reviews and improves the MongoDB connection strategy used in mongodb.ts to better support serverless environments.

Changes made

• Refactored database connection initialization to avoid potential connection leaks.
• Improved connection reuse logic for serverless function invocations.
• Reduced risk of connection pool exhaustion during concurrent requests.
• Added safeguards around connection state management.
• Aligned the implementation with serverless deployment best practices.

Pillar

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

Visual Preview

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.

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