feat: wire up mic/speaker and Google Translate backend for live translation

Frontend uses the Web Speech API for speech-to-text and text-to-speech
(no key needed). Adds POST /api/translate backed by Google Cloud
Translation API v2, gated on GOOGLE_TRANSLATE_API_KEY (placeholder
added to .env/.env.example, unset by default).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/Backend/.env.example

@@ -10,4 +10,5 @@ DB_HOST=localhost
 DB_USER=postgres
 DB_PASSWORD=mypassword
 DB_NAME=dentalapp
-DATABASE_URL=postgresql://postgres:mypassword@localhost:5432/dentalapp
+DATABASE_URL=postgresql://postgres:mypassword@localhost:5432/dentalapp
+GOOGLE_TRANSLATE_API_KEY=