fix(oauth): set GOOGLE_REDIRECT_URI in docker-compose; guard missing credentials

- Add APP_URL and GOOGLE_REDIRECT_URI=https://scoreodonto.com/api/oauth2callback
  to docker-compose backend environment
- /api/auth/google/url returns 503 with clear message when CLIENT_ID/SECRET unset
- GoogleConnectButton shows alert instead of silently breaking on API error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/server.js

@@ -356,6 +356,9 @@ function schedulePush(tabName) {
 
 // --- GOOGLE AUTH ROUTES ---
 app.get('/api/auth/google/url', (req, res) => {
+    if (!process.env.GOOGLE_CLIENT_ID || !process.env.GOOGLE_CLIENT_SECRET) {
+        return res.status(503).json({ error: 'GOOGLE_CLIENT_ID e GOOGLE_CLIENT_SECRET não configurados no servidor.' });
+    }
     const { dentistId } = req.query;
     const oauth2Client = createOAuth2Client();
     const url = oauth2Client.generateAuthUrl({