Add user-configurable TTS pronunciation overrides

A JSON map (TTS_PRONUNCIATIONS_PATH, default tts_pronunciations.json)
rewrites mispronounced words — place names, initialisms, dotted
abbreviations — to phonetic spellings before synthesis, applied after
markdown cleanup in both /tts/speech paths. Whole-word smartcase
matching (lowercase keys match any casing, uppercase keys exact),
longest key wins, hot-reloaded on mtime change with last-good fallback
on parse errors. See tts_pronunciations.example.json.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ai/mod.rs

@@ -12,6 +12,7 @@ pub mod llm_client;
 pub mod local_llm;
 pub mod ollama;
 pub mod openrouter;
+pub mod pronunciation;
 pub mod sms_client;
 pub mod tts;
 pub mod turn_registry;