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>

app/config.py

@@ -42,6 +42,13 @@ class Settings(BaseSettings):
     chat_agent_timeout_s: float = 25.0
     chat_agent_max_steps: int = 5
 
+    # Voice input (#116): speech-to-text for the patient's messages. `auto` = OpenRouter if a key is set, else mock.
+    # OpenRouter STT is synthetic data only (ADR-014): the audio leaves the RF and cannot be scrubbed.
+    stt_provider: Literal["auto", "off", "mock", "openrouter"] = "auto"
+    stt_model: str = "google/gemini-2.5-flash"  # needs audio input
+    stt_timeout_s: float = 30.0
+    stt_max_seconds: int = Field(default=60, ge=5, le=300)
+
     # Telegram is used only for the prototype on synthetic data; prod channel is MAX / Gosuslugi.
     tg_bot_token: str = ""
     # The RF server reaches api.telegram.org over IPv6 only; docker has IPv4 only → go through a proxy (http/socks).