feat: improve user schema validation and serialization

[jikrana1]

🛠 Pull Request Template

📌 Related Issue

Fixes: #388

---

🔍 Describe your changes?

• Added email format validation to prevent invalid email addresses.
• Added a maximum length constraint for the name field.
• Configured googleId and githubId with unique and sparse indexes to avoid duplicate OAuth identifiers.
• Added a toJSON transform to exclude sensitive fields from API responses.
• Improved overall user schema structure for better validation and production readiness.
• Maintained backward compatibility with the existing authentication flow.

---

🧪 Checklist

Please check all that apply:

• I have tested my changes locally.
• I have followed the project's code style and guidelines.
• I have added necessary comments and documentation.
• The code compiles and runs without errors.

---

Program:

SSOC 2026

---

🗒️ Additional Notes (Optional)

• These changes improve schema validation and data consistency.
• Sensitive fields are excluded from JSON responses for safer API serialization.
• Existing login and authentication flow remain unaffected.

---

Thank you for contributing!

---

[vercel]

Someone is attempting to deploy a commit to the niharika-mente's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Aamod-Dev]

The user schema validation improvements (email regex, name maxlength, sparse indexes, toJSON transform) look good — they improve data integrity and security.

However, the FRONTEND/package-lock.json changes are concerning: you have removed peer: true from many entries (chakra, emotion, react, eslint, vitest, testing-library) and added it to others. This can break dependency resolution for contributors and in CI. Please revert the lockfile to its original state — the schema changes do not require any lockfile modifications.