Title: Add refresh token rotation to auth serviceLabels: enhancement security

[Uyoxy]

Description

src/modules/auth/auth.service.ts lacks refresh token rotation.
Stolen refresh tokens remain valid indefinitely without rotation,
creating a security risk for all users.

Implementation Requirements

• Update src/modules/auth/auth.service.ts to implement rotating refresh tokens
• Add refreshToken and refreshTokenExpiry fields to src/modules/users/user.entity.ts
• Add POST /auth/refresh endpoint in src/modules/auth/auth.controller.ts
• Invalidate old refresh token after each use
• Add unit tests in src/modules/auth/auth.service.spec.ts

Acceptance Criteria

• POST /auth/refresh returns new access and refresh tokens
• Old refresh token invalidated after use
• Reusing old refresh token returns 401
• Unit tests pass
• npm run lint passes
• CI checks pass before PR is merged

Evidence Required

Contributors must provide ALL of the following:

• Screenshot of server running with npm run start:dev
• Screenshot of Postman or curl showing POST /auth/refresh returning new tokens
• Screenshot of reused token returning 401
• Screenshot of passing CI checks on the PR

Files Affected

• src/modules/auth/auth.service.ts (modify)
• src/modules/auth/auth.controller.ts (modify)
• src/modules/users/user.entity.ts (modify)
• src/modules/auth/auth.service.spec.ts (create)

[nafiuishaaq]

Let me professionally implement this task as soon as possible with an ETA of 2 to 4 hours

[grantfox-oss]

🦊 GrantFox — @nafiuishaaq has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #68)
2. Your PR will be reviewed by the stellarspend maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@nafiuishaaq's PR #100 was approved and merged by @Uyoxy.

🏆 @nafiuishaaq: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (118 total points). Track your full progress on GrantFox.

👏 Great work, @nafiuishaaq! Keep contributing to stellarspend.