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/webui/settings_api.py

@@ -91,7 +91,7 @@ _IMAGE_GENERATION_ASPECT_RATIOS = {
     "2:3",
     "21:9",
 }
-_TRANSCRIPTION_PROVIDERS = ("groq", "openai")
+_TRANSCRIPTION_PROVIDERS = ("groq", "openai", "openrouter")
 _CONTEXT_WINDOW_TOKEN_OPTIONS = {65_536, 262_144}
 _MODEL_CONFIGURATION_SLUG_RE = re.compile(r"[^a-z0-9_-]+")
 _ENV_REF_RE = re.compile(r"\$\{([A-Za-z_][A-Za-z0-9_]*)\}")