fix: consolidate Google OAuth dependency

[atomantic]

Closes #7013

Summary

• remove the redundant direct server dependency on google-auth-library
• source OAuth2Client through @googleapis/calendar's existing auth.OAuth2 export
• update the server lockfile and dependency audit trail

Validation

• focused server tests: 50 passed (docs/deps-doc.test.js, calendar route/account/sync suites)
• OAuth2 export and googleAuth module-load smoke checks passed
• full server suite: 2,161 passed, 7 unrelated failures involving disk-full/timeouts in peer-sync/eidoverse, JSON writeback, voice, Beeper, and video-generation tests
• required Claude medium-effort read-only review: CLEAN