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.
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