feat: add GitLab OAuth sign-in and token storage

[sanrishi]

Closes #9

Adds GitLab as a second OAuth provider alongside GitHub.

Changes Made

• next-auth.d.ts — added gitlabToken, gitlabId, gitlabLogin to Session and JWT interfaces
• auth.ts — added GitLabProvider, GitLab branches in signIn/jwt/session callbacks
• schema.sql — added gitlab_id, gitlab_login columns, made github_id nullable
• page.tsx — added "Sign in with GitLab" button

How to Test

1. Add GITLAB_ID and GITLAB_SECRET to .env.local
2. Run npm run dev
3. Click "Sign in with GitLab" — should reach GitLab consent
4. After sign-in, session.gitlabToken should be set
5. GitHub sign-in still works unchanged

[vercel]

@sanrishi is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[sanrishi]

@Priyanshu-byte-coder This also ready for review!