Switch AudioButton to Piper TTS (Thorsten/Lessac voices)

AudioButton now tries Piper TTS via /api/vocabulary/tts endpoint
first, falls back to Browser Web Speech API if unavailable.

Backend: New GET /api/vocabulary/tts?text=...&lang=de endpoint.
audio_service.py: Fixed presigned URL flow for MinIO download.

This gives the same high-quality voice as the Investor Agent
in the pitch deck (Thorsten DE / Lessac EN, MIT license).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/services/audio.py

@@ -74,16 +74,24 @@ async def synthesize_word(
                 return None
 
             data = resp.json()
-            audio_url = data.get("audio_url") or data.get("presigned_url")
+            bucket = data.get("bucket")
+            object_key = data.get("object_key")
 
-            if audio_url:
-                # Download the audio file
-                audio_resp = await client.get(audio_url)
-                if audio_resp.status_code == 200:
-                    with open(cached, "wb") as f:
-                        f.write(audio_resp.content)
-                    logger.info(f"TTS cached: '{text}' ({language}) → {cached}")
-                    return cached
+            if bucket and object_key:
+                # Get presigned URL to download the audio
+                url_resp = await client.post(
+                    f"{TTS_SERVICE_URL}/presigned-url",
+                    json={"bucket": bucket, "object_key": object_key, "expires": 300},
+                )
+                if url_resp.status_code == 200:
+                    audio_url = url_resp.json().get("url")
+                    if audio_url:
+                        audio_resp = await client.get(audio_url)
+                        if audio_resp.status_code == 200:
+                            with open(cached, "wb") as f:
+                                f.write(audio_resp.content)
+                            logger.info(f"TTS cached: '{text}' ({language}) → {cached}")
+                            return cached
 
     except Exception as e:
         logger.warning(f"TTS service unavailable: {e}")