feat(llm): speech-to-text for patient voice input via OpenRouter (+ offline mock)

POST /api/v1/me/{token}/stt takes a 16 kHz mono WAV and only transcribes:
the patient checks the text and sends it as an ordinary message, so safety,
scrub() and the LLM gateway still apply. Audio is neither stored nor logged.
OpenRouter STT is synthetic-data only (ADR-014); SaluteSpeech for the pilot is #121.

Refs #116

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -11,6 +11,10 @@ CHAT_AGENT=auto
 OPENROUTER_API_KEY=
 OPENROUTER_MODEL=google/gemini-2.5-flash-lite
 
+# --- Voice input (speech-to-text): auto | off | mock | openrouter. OpenRouter = synthetic data only (ADR-014).
+STT_PROVIDER=auto
+STT_MODEL=google/gemini-2.5-flash
+
 # --- Pseudonymisation key: 32+ random bytes, hex. Generate: python -c "import secrets;print(secrets.token_hex(32))"
 PSEUDO_KEY=