Refactor OAuth implementation and enhance account linking features

[hhhhsc701]

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[codecov]

Codecov Report

❌ Patch coverage is 74.56647% with 176 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/services/oauth_service.py	44.06%	96 Missing and 3 partials ⚠️
backend/apps/oauth_app.py	68.65%	36 Missing and 6 partials ⚠️
backend/consts/oauth_providers.py	0.00%	17 Missing ⚠️
backend/database/oauth_account_db.py	74.50%	13 Missing ⚠️
backend/utils/auth_utils.py	76.19%	3 Missing and 2 partials ⚠️

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