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>

tts_pronunciations.example.json

@@ -0,0 +1,13 @@
+{
+  "Worcester": "Wuster",
+  "Spokane": "Spo can",
+  "wsl": "W S L",
+  "sql": "sequel",
+  "api": "A P I",
+  "US": "U S",
+  "Dr.": "Doctor",
+  "St.": "Saint",
+  "blvd": "boulevard",
+  "vs.": "versus",
+  "etc.": "et cetera"
+}