Add TTS abbreviation expansion (sth→something, sb→somebody, etc.)

Text is preprocessed before TTS to expand abbreviations like
sth., sb., etc., z.B., usw. so the speaker says the full word.

40+ abbreviations covered (EN + DE). Applied to all languages.
Audio cache cleared to regenerate with correct pronunciation.

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

backend-lehrer/services/audio.py

@@ -23,6 +23,53 @@ 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")
 
+# Abbreviations expanded before TTS (so the speaker says the full word)
+_TTS_EXPANSIONS = {
+    "sth.": "something",
+    "sth": "something",
+    "sb.": "somebody",
+    "sb": "somebody",
+    "smth.": "something",
+    "smb.": "somebody",
+    "sbd.": "somebody",
+    "etc.": "etcetera",
+    "e.g.": "for example",
+    "i.e.": "that is",
+    "esp.": "especially",
+    "approx.": "approximately",
+    "vs.": "versus",
+    "nr.": "number",
+    "no.": "number",
+    "p.": "page",
+    "adj.": "adjective",
+    "adv.": "adverb",
+    "prep.": "preposition",
+    "pron.": "pronoun",
+    "pl.": "plural",
+    "sg.": "singular",
+    "syn.": "synonym",
+    "ant.": "antonym",
+    # DE
+    "usw.": "und so weiter",
+    "bzw.": "beziehungsweise",
+    "z.B.": "zum Beispiel",
+    "d.h.": "das heisst",
+    "vgl.": "vergleiche",
+    "ca.": "circa",
+    "evtl.": "eventuell",
+    "ggf.": "gegebenenfalls",
+}
+
+
+def _expand_abbreviations(text: str) -> str:
+    """Expand abbreviations so TTS speaks the full word."""
+    import re
+    for abbr, full in _TTS_EXPANSIONS.items():
+        # Word-boundary aware replacement (case-insensitive)
+        pattern = re.escape(abbr)
+        text = re.sub(rf'\b{pattern}', full, text, flags=re.IGNORECASE)
+    return text
+
 
 def _ensure_cache_dir():
     os.makedirs(AUDIO_CACHE_DIR, exist_ok=True)
@@ -56,13 +103,16 @@ async def synthesize_word(
     if os.path.exists(cached):
         return cached
 
+    # Expand abbreviations before speaking
+    speak_text = _expand_abbreviations(text)
+
     # Call Piper TTS service via /synthesize-direct (returns MP3, selects language correctly)
     try:
         async with httpx.AsyncClient(timeout=30.0) as client:
             resp = await client.post(
                 f"{TTS_SERVICE_URL}/synthesize-direct",
                 json={
-                    "text": text,
+                    "text": speak_text,
                     "language": language,
                 },
             )