feat: add configurable vision focus prompts

backend/models.py

@@ -92,6 +92,7 @@ class AssistantConfig(BaseModel):
     llm_support_image_input: bool = False
     vision_enabled: bool = False
     vision_model_resource_id: str | None = None
+    vision_focus_prompt: str = ""
     vision_model: str = ""
     vision_llm_interface_type: str = "openai-llm"
     vision_llm_values: dict = {}