Build Authentication API (/auth/login, /auth/callback, /auth/logout)

[roryplans-ai]

Parent Issue: #5
PlanId: 7494ee3e-3e62-4df1-9c16-932a5004c59d
CardId: GAR2
Subtask ID: GAR2.2.1

Description: Create the necessary endpoints to handle the OAuth 2.0 authentication flow, including login, callback, and logout functionalities.