feat(auth): restore Google sign-in and AI admin test routes

Google OAuth was implemented locally but never deployed; the login button
only renders when AUTH_GOOGLE_ID and AUTH_GOOGLE_SECRET are set. Also
restores /api/ai/test-* endpoints removed by mistake and wires Google
credentials into deploy workflows.

Co-authored-by: Cursor <cursoragent@cursor.com>

memento-note/.env.example

@@ -19,6 +19,12 @@ NEXTAUTH_URL="http://localhost:3000"
 # Admin email - The first user registering with this email gets ADMIN role (REQUIRED)
 # ADMIN_EMAIL="admin@yourdomain.com"
 
+# Google OAuth (optional — shows "Continue with Google" on login when both are set)
+# Create credentials: https://console.cloud.google.com/apis/credentials
+# Authorized redirect URI: {NEXTAUTH_URL}/api/auth/callback/google
+# AUTH_GOOGLE_ID="....apps.googleusercontent.com"
+# AUTH_GOOGLE_SECRET="GOCSPX-..."
+
 # -----------------------------------------------------------------------------
 # AI Providers
 # -----------------------------------------------------------------------------