feat(auth): add GitLab OAuth login and harden Google/GitHub sign-in

[parth0025]

What changed

Adds GitLab as a third social-login provider, and fixes the existing Google/GitHub login so server-side verification actually runs.

GitLab (new)

• Modules/gitlabOAuth/ — authorization-code → token exchange (sends the redirect_uri GitLab requires).
• verifyGitlabAuth in authHelpers.js — re-verifies the identity against GitLab /user before issuing a session; gitlab-signup endpoint for invitation signups; gitlabId added to the userAuth schema.
• Frontend GitlabAuth.vue + tanuki icon, wired into the provider list (gated by VUE_APP_IS_GITLAB_LOGIN).
• Self-managed GitLab supported via GITLAB_BASE_OAUTH_URL / GITLAB_BASE_API_URL.

Google / GitHub (fix)

• The client now forwards the Google id-token and the GitHub access token, so the hardened backend verification path runs instead of rejecting every login.
• The Google verifier accepts a single configured client id (GOOGLE_CLIENT_ID) rather than the orphaned GOOGLE_OAUTH_CLIENT_ID.
• GitHub and GitLab both return via ?code=; each redirect handler now ignores the other provider''s code so they no longer collide.

Docs / tests

• .env.example documents every OAuth var for all three providers.
• .claude/test-cases/OAuthGoogleLogin.md, OAuthGithubLogin.md, OAuthGitlabLogin.md — manual test cases with the provider app-configuration steps, console URLs, and env-var tables.

Verification

• ESLint clean on all changed frontend files; backend modules runtime-load with STORAGE_TYPE set; index.js syntax OK; jest suite passes.
• Manually tested in the browser: Google, GitHub, and GitLab login all working (per the user); the three buttons render together.

No secrets are committed — .env / frontend/.env are gitignored; only the empty-placeholder .env.example is included.

🤖 Generated with Claude Code

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: e6954ffd-f0a2-4fdd-8851-0da4bff09099

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/oauth-gitlab

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}