feat(transcription): configurable STT model + OpenRouter provider

Add a `transcriptionModel` channel setting and an OpenRouter transcription
backend so voice messages can be transcribed through OpenRouter's
speech-to-text endpoint (e.g. nvidia/parakeet-tdt-0.6b-v3, openai/whisper-1),
alongside the existing Groq/OpenAI Whisper providers.

- schema: add channels.transcriptionModel (None = provider default)
- providers/transcription: extract a shared POST/retry skeleton; add a
  JSON+base64 OpenRouterTranscriptionProvider; make the STT model a
  constructor param on all providers instead of hardcoding it
- channels: route transcriptionProvider="openrouter" and thread the model
  through the manager to each channel
- docs + tests

Only dedicated STT models work on OpenRouter's transcription endpoint;
chat LLMs (e.g. google/gemini-3.5-flash) are rejected there.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nanobot/config/schema.py

@@ -47,7 +47,7 @@ class TranscriptionConfig(Base):
     """Cross-channel audio transcription configuration."""
 
     enabled: bool = True
-    provider: Literal["groq", "openai"] | None = None
+    provider: Literal["groq", "openai", "openrouter"] | None = None
     model: str | None = None
     language: str | None = Field(default=None, pattern=r"^[a-z]{2,3}$")
     max_duration_sec: int = Field(default=120, ge=1, le=600)