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/audio_service.py

@@ -0,0 +1,125 @@
+"""
+Audio Service — Generates TTS audio for vocabulary words.
+
+Uses the Piper TTS service (compliance-tts-service, MIT license)
+for high-quality German (Thorsten) and English (Lessac) voices.
+Falls back to a placeholder response if TTS service is unavailable.
+
+Audio files are cached — generated once, served forever.
+"""
+
+import hashlib
+import logging
+import os
+from typing import Optional
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+# Piper TTS service (runs in compliance stack)
+TTS_SERVICE_URL = os.getenv("TTS_SERVICE_URL", "http://bp-compliance-tts:8095")
+
+# Local cache directory for generated audio
+AUDIO_CACHE_DIR = os.path.expanduser("~/Arbeitsblaetter/audio-cache")
+
+
+def _ensure_cache_dir():
+    os.makedirs(AUDIO_CACHE_DIR, exist_ok=True)
+
+
+def _cache_key(text: str, language: str) -> str:
+    """Generate a deterministic cache key for text + language."""
+    h = hashlib.sha256(f"{language}:{text}".encode()).hexdigest()[:16]
+    return f"{language}_{h}"
+
+
+def _cache_path(text: str, language: str) -> str:
+    """Full path to cached MP3 file."""
+    _ensure_cache_dir()
+    return os.path.join(AUDIO_CACHE_DIR, f"{_cache_key(text, language)}.mp3")
+
+
+async def synthesize_word(
+    text: str,
+    language: str = "de",
+    word_id: str = "",
+) -> Optional[str]:
+    """
+    Generate TTS audio for a word or short phrase.
+
+    Returns the file path to the cached MP3, or None on error.
+    Uses Piper TTS service (compliance-tts-service).
+    """
+    # Check cache first
+    cached = _cache_path(text, language)
+    if os.path.exists(cached):
+        return cached
+
+    # Call Piper TTS service
+    try:
+        async with httpx.AsyncClient(timeout=30.0) as client:
+            resp = await client.post(
+                f"{TTS_SERVICE_URL}/synthesize",
+                json={
+                    "text": text,
+                    "language": language,
+                    "voice": "thorsten-high" if language == "de" else "lessac-high",
+                    "module_id": "vocabulary",
+                    "content_id": word_id or _cache_key(text, language),
+                },
+            )
+            if resp.status_code != 200:
+                logger.warning(f"TTS service returned {resp.status_code} for '{text}'")
+                return None
+
+            data = resp.json()
+            audio_url = data.get("audio_url") or data.get("presigned_url")
+
+            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
+
+    except Exception as e:
+        logger.warning(f"TTS service unavailable: {e}")
+
+    # Fallback: try direct MP3 endpoint
+    try:
+        async with httpx.AsyncClient(timeout=30.0) as client:
+            resp = await client.post(
+                f"{TTS_SERVICE_URL}/synthesize/mp3",
+                json={
+                    "text": text,
+                    "language": language,
+                    "voice": "thorsten-high" if language == "de" else "lessac-high",
+                    "module_id": "vocabulary",
+                },
+            )
+            if resp.status_code == 200 and resp.headers.get("content-type", "").startswith("audio"):
+                with open(cached, "wb") as f:
+                    f.write(resp.content)
+                logger.info(f"TTS cached (direct): '{text}' ({language}) → {cached}")
+                return cached
+    except Exception as e:
+        logger.debug(f"TTS direct fallback also failed: {e}")
+
+    return None
+
+
+async def get_or_generate_audio(
+    text: str, language: str = "de", word_id: str = "",
+) -> Optional[bytes]:
+    """
+    Get audio bytes for a word. Returns MP3 bytes or None.
+    Generates via TTS if not cached.
+    """
+    path = await synthesize_word(text, language, word_id)
+    if path and os.path.exists(path):
+        with open(path, "rb") as f:
+            return f.read()
+    return None