feat: Revamp OAuth integration to utilize n8n for account connections

• Updated OAuth endpoints to connect accounts directly via n8n, eliminating the need for popups.
• Enhanced oauth_connect endpoint to test n8n connections and create accounts in the database.
• Removed obsolete OAuth authorization and callback logic, streamlining the integration process.
• Updated OAuthService to manage connections and handle n8n-specific logic.
• Improved documentation and comments for clarity regarding the new workflow.