feat(citation): support downloading cited documents with allow_download toggle

Added `allow_download` flag to citation config and `download_url` field to citation output. Implemented `/citations/{document_id}/download` endpoint to serve original files when enabled. Removed unused `files` field and `HttpRequestDataProcessing` model from HTTP request node config.

api/app/core/workflow/nodes/http_request/config.py

@@ -132,11 +132,6 @@ class HttpErrorDefaultTemplate(BaseModel):
         description="Default HTTP headers returned on error",
     )
 
-    files: list = Field(
-        default_factory=list,
-        description="Default files list returned on error",
-    )
-
     output: str = Field(
         default="SUCCESS",
         description="HTTP response body",
@@ -251,13 +246,6 @@ class HttpRequestNodeConfig(BaseNodeConfig):
         }
 
 
-class HttpRequestDataProcessing(BaseModel):
-    request: str = Field(
-        default="",
-        description="Raw HTTP request format for debugging",
-    )
-
-
 class HttpRequestNodeOutput(BaseModel):
     body: str = Field(
         ...,