WIP: Add GitLab platform OAuth config

[simple-agent-manager]

WIP status

This is intentionally a draft/WIP PR. Do not deploy to staging and do not merge to production for this task. Staging verification was explicitly skipped.

Summary

• Adds GitLab OAuth to the DB-backed platform integration config model with runtime admin/setup storage and GITLAB_* env fallback.
• Wires GitLab into BetterAuth login provider config plus landing, device auth, trial login, setup, and admin platform config UI surfaces.
• Updates deploy/env examples and public docs for optional GitLab config, with encrypted runtime storage for the client secret.
• Adds resolver, validation, setup/admin route, auth provider, login provider, UI unit, and Playwright audit coverage.

Validation

• pnpm test passed on the final branch: 19 Turbo tasks, API 391 files / 5860 tests, web 216 files / 2643 tests.
• pnpm lint passed with existing warning noise and no errors.
• pnpm typecheck passed.
• pnpm build passed.
• npx playwright test tests/playwright/platform-config-audit.spec.ts --project="iPhone SE (375x667)" --project="Desktop (1280x800)" passed.

Staging

Skipped by explicit instruction. No staging deploy, staging mutation, production deploy, or merge was performed.

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.7% Duplication on New Code

See analysis details on SonarQube Cloud

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing sam/gitlab-platform-config-wip (7220fae) with main (3107069)}