Add Piper TTS audio integration for vocabulary words

audio_service.py: Connects to compliance-tts-service (Piper TTS,
MIT license) for high-quality German (Thorsten) and English (Lessac)
voices. Audio cached as MP3 on first request.

vocabulary_api.py: New endpoints:
- GET /vocabulary/word/{id}/audio/{lang} — word pronunciation
- GET /vocabulary/word/{id}/audio-syllables/{lang} — slow syllable-by-syllable

Anton App analysis: identified 5 features to adopt (star system,
games as rewards, progress bars, listening exercises, matching exercises).

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

backend-lehrer/vocabulary_api.py

@@ -99,6 +99,68 @@ async def api_get_filters():
     }
 
 
+# ---------------------------------------------------------------------------
+# Audio TTS for Words
+# ---------------------------------------------------------------------------
+
+
+@router.get("/word/{word_id}/audio/{lang}")
+async def api_get_word_audio(word_id: str, lang: str = "en"):
+    """Get or generate TTS audio for a vocabulary word.
+
+    Returns MP3 audio. Generated on first request, cached after.
+    Uses Piper TTS (MIT license) with Thorsten (DE) and Lessac (EN) voices.
+    """
+    from fastapi.responses import Response as FastAPIResponse
+
+    word = await get_word(word_id)
+    if not word:
+        raise HTTPException(status_code=404, detail="Wort nicht gefunden")
+
+    text = word.english if lang == "en" else word.german
+    if not text:
+        raise HTTPException(status_code=400, detail=f"Kein Text fuer Sprache '{lang}'")
+
+    from audio_service import get_or_generate_audio
+    audio_bytes = await get_or_generate_audio(text, language=lang, word_id=word_id)
+
+    if not audio_bytes:
+        raise HTTPException(status_code=503, detail="TTS Service nicht verfuegbar")
+
+    return FastAPIResponse(content=audio_bytes, media_type="audio/mpeg")
+
+
+@router.get("/word/{word_id}/audio-syllables/{lang}")
+async def api_get_syllable_audio(word_id: str, lang: str = "en"):
+    """Get TTS audio with slow syllable pronunciation.
+
+    Generates audio like "ap ... ple" with pauses between syllables.
+    """
+    from fastapi.responses import Response as FastAPIResponse
+
+    word = await get_word(word_id)
+    if not word:
+        raise HTTPException(status_code=404, detail="Wort nicht gefunden")
+
+    syllables = word.syllables_en if lang == "en" else word.syllables_de
+    if not syllables:
+        # Fallback to full word
+        text = word.english if lang == "en" else word.german
+        syllables = [text]
+
+    # Join syllables with pauses (Piper handles "..." as pause)
+    slow_text = " ... ".join(syllables)
+
+    from audio_service import get_or_generate_audio
+    cache_key = f"{word_id}_syl_{lang}"
+    audio_bytes = await get_or_generate_audio(slow_text, language=lang, word_id=cache_key)
+
+    if not audio_bytes:
+        raise HTTPException(status_code=503, detail="TTS Service nicht verfuegbar")
+
+    return FastAPIResponse(content=audio_bytes, media_type="audio/mpeg")
+
+
 # ---------------------------------------------------------------------------
 # Learning Unit Creation from Word Selection
 # ---------------------------------------------------------------------------