TTS: include mp3 files in cache stats

TTSService._put_cache writes .mp3 for MP3 audio (ID3/MPEG-framed bytes) and
.wav otherwise, and the rest of the class treats both as cache entries
(_get_cache iterates (".mp3", ".wav"); eviction globs "*.*"). But
get_stats() enumerated the cache with `glob("*.wav")` only, so both
cache_entries and cache_size_mb undercounted — reporting 0 whenever the
cache held MP3 files, which is the common case for most TTS providers.

Glob both extensions so the reported stats match what's actually cached.

tests/test_tts_cache_stats.py writes an MP3-headed blob via _put_cache and
asserts get_stats() reports one entry with non-zero size. Fails before this
change.

services/tts/tts_service.py

@@ -188,7 +188,7 @@ class TTSService:
         provider = settings["tts_provider"]
         tts_enabled = settings.get("tts_enabled", True)
 
-        cache_files = list(self.cache_dir.glob("*.wav"))
+        cache_files = list(self.cache_dir.glob("*.wav")) + list(self.cache_dir.glob("*.mp3"))
         cache_size = sum(f.stat().st_size for f in cache_files)
 
         is_available = self.available and tts_enabled