[codex] Add analytics preference API

[jjoonleo]

Summary

• Add account-scoped analytics preference persistence and GET/PUT /users/me/analytics-preference endpoints for issue feat: add account-scoped analytics preference API #318.
• Gate the backend default behind ANALYTICS_PREFERENCE_DEFAULT_ENABLED=false until privacy policy and Data Safety approval.
• Create default preference rows during normal, Google, Apple, and Kakao registration and repair missing rows on read.
• Document the API, update Swagger examples, and include deletion cascade behavior in account deletion docs.

Validation

• ./gradlew test
• git diff --check

Notes

• Hosted privacy policy copy is intentionally unchanged; that remains scoped to issue chore: update hosted privacy policy for Firebase Analytics release #319.