feat(transcription): add Xiaomi MiMo ASR provider (mimo-v2.5-asr)

Add support for Xiaomi MiMo ASR as a third transcription backend alongside
Groq and OpenAI Whisper. Xiaomi ASR uses the /v1/chat/completions endpoint
with base64-encoded audio input, rather than the standard Whisper multipart
upload format.

Co-Authored-By:连 <lian@tangping.homes>

nanobot/config/schema.py

@@ -47,7 +47,7 @@ class TranscriptionConfig(Base):
     """Cross-channel audio transcription configuration."""
 
     enabled: bool = True
-    provider: Literal["groq", "openai", "openrouter"] | None = None
+    provider: Literal["groq", "openai", "openrouter", "xiaomi_mimo"] | 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)