F1: add reusable task backend resources and migrate task tools

backend/models.py

@@ -70,6 +70,21 @@ class RuntimeKnowledgeBase(BaseModel):
     description: str = ""
 
 
+class RuntimeTaskBackend(BaseModel):
+    """Resolved task backend, frozen server-side when a task is accepted."""
+
+    id: str
+    name: str = ""
+    type: Literal["reasoner", "opencode"]
+    config: dict = Field(default_factory=dict)
+    secrets: dict = Field(default_factory=dict)
+    model: RuntimeModelResource | None = None
+    tools: list[RuntimeTool] = Field(default_factory=list)
+    tool_bindings: list[RuntimeToolBinding] = Field(default_factory=list)
+    instruction_source: dict = Field(default_factory=lambda: {"source": "model"})
+    instructions: str = ""
+
+
 class AssistantConfig(BaseModel):
     """运行时配置:前端可见部分(name/prompt/...) + 服务端注入部分(*_api_key/*_base_url)。"""
 
@@ -138,6 +153,7 @@ class AssistantConfig(BaseModel):
     # and result mappings; platform built-ins are configured separately.
     tools: list[RuntimeTool] = Field(default_factory=list)
     tool_bindings: list[RuntimeToolBinding] = Field(default_factory=list)
+    task_backend: RuntimeTaskBackend | None = None
     knowledge_base_id: str | None = None
     knowledge_base_name: str = ""
     knowledge_base_description: str = ""