feat(auth): add Grantfox OAuth integration

[devIKargi]

Closes #144

Description

Adds Grantfox OAuth2 SSO integration using the PKCE flow.

Changes include:

• Grantfox OAuth authorization, callback, token exchange, refresh, and revoke flows
• Grantfox controller, OAuth service, and identity middleware
• Encrypted refresh-token persistence
• Grantfox DTOs and database entity
• Environment configuration and validation
• grantfox social provider support

Fixes #

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Performance improvement
• Code refactoring

How Has This Been Tested?

• Unit tests added/updated
• E2E tests added/updated
• Manual testing performed
• Tested in staging environment

Validation performed:

2 test suites passed
26 tests passed
npm run build passed

Reproduce the focused tests with:

npx jest src/core/auth/grantfox.controller.spec.ts src/core/auth/services/grantfox-oauth.service.spec.ts --runInBand

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review
• I have added tests that prove the feature works
• New and existing relevant unit tests pass locally
• I have made corresponding changes to the documentation
• Tested in staging environment
• Dependent changes have been merged and published downstream

Screenshots

Not applicable.

Additional Notes

Grantfox credentials and encryption settings are documented in .env.example. The integration requires valid Grantfox OAuth client configuration and a 32-byte encryption key for refresh-token storage.