feat(auth): improve Google OAuth validation and language synchronization

[aniebietafia]

• Add flow parameter (login | signup) to Google OAuth login flow.
• Add validations preventing OAuth auto-linking for email/password users and preventing login for non-existent users.
• Add is_new_user boolean field to the POST /google/exchange response payload.
• Synchronize speaking and listening languages on registration and profile updates.
• Add integration and unit tests for OAuth flow validations and language synchronization behavior.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f514ba63-6476-4546-a17f-f373099b5394

📥 Commits

Reviewing files that changed from the base of the PR and between cf9ecd8 and 0d818cb.

📒 Files selected for processing (10)

• app/modules/auth/api_docs.md
• app/modules/auth/router.py
• app/modules/auth/schemas.py
• app/modules/auth/service.py
• app/modules/user/api_docs.md
• app/modules/user/router.py
• tests/test_auth/test_auth_signup.py
• tests/test_auth/test_oauth_google.py
• tests/test_auth/test_oauth_service.py
• tests/test_user/test_user_endpoints.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fixes/google_oauth

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