feat(auth): add Google OAuth routes for CostGuard billing

main.py

@@ -93,6 +93,39 @@ oauth.register(
     }
 )
 
+
+@app.get('/auth/login')
+async def login(request: Request):
+    """Redirects user to Google's OAuth 2.0 login page."""
+    redirect_uri = "https://opax.vauco.no/auth/callback"
+    return await oauth.google.authorize_redirect(request, redirect_uri)
+
+@app.get('/auth/callback', name='auth')
+async def auth(request: Request):
+    """
+    Handles the callback from Google's OAuth.
+    Stores user info in session and redirects to the billing dashboard.
+    """
+    token = await oauth.google.authorize_access_token(request)
+    user = token.get('userinfo')
+    if user:
+        request.session['user'] = dict(user)
+    return RedirectResponse(url='/static/billing-dashboard.html')
+
+@app.get('/auth/me')
+@require_auth
+async def me(request: Request):
+    """Returns the authenticated user's information."""
+    user = request.session.get('user')
+    return JSONResponse(user)
+
+@app.get('/auth/logout')
+async def logout(request: Request):
+    """Clears the user session and logs them out."""
+    request.session.pop('user', None)
+    return RedirectResponse(url='/static/billing-dashboard.html')
+
+
 # Allowed emails for login
 ALLOWED_EMAILS = [email.strip() for email in os.environ.get("ALLOWED_EMAILS", "").split(",") if email.strip()]
 ALERT_EMAIL = os.environ.get("ALERT_EMAIL")