Fix wrong-prefix silent ignore and Minimax image understand host

- Remove "Use .cmd for this role" reply when owner uses wrong prefix;
  wrong-prefix commands are now silently ignored for all users
- Fix AIImageUnderstandConfig default api_host to api.minimax.chat
  (minimax-coding-plan-mcp routes all calls there, not api.minimax.io)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/config.py

@@ -51,7 +51,7 @@ class AIModelConfig:
 @dataclass(slots=True)
 class AIImageUnderstandConfig:
     api_key: str
-    api_host: str = "https://api.minimax.io"
+    api_host: str = "https://api.minimax.chat"
     command: tuple[str, ...] = ("uvx", "minimax-coding-plan-mcp", "-y")
     timeout: float | None = None