Initial commit: hardened DeerFlow factory

Vendored deer-flow upstream (bytedance/deer-flow) plus prompt-injection
hardening:

- New deerflow.security package: content_delimiter, html_cleaner,
  sanitizer (8 layers — invisible chars, control chars, symbols, NFC,
  PUA, tag chars, horizontal whitespace collapse with newline/tab
  preservation, length cap)
- New deerflow.community.searx package: web_search, web_fetch,
  image_search backed by a private SearX instance, every external
  string sanitized and wrapped in <<<EXTERNAL_UNTRUSTED_CONTENT>>>
  delimiters
- All native community web providers (ddg_search, tavily, exa,
  firecrawl, jina_ai, infoquest, image_search) replaced with hard-fail
  stubs that raise NativeWebToolDisabledError at import time, so a
  misconfigured tool.use path fails loud rather than silently falling
  back to unsanitized output
- Native client back-doors (jina_client.py, infoquest_client.py)
  stubbed too
- Native-tool tests quarantined under tests/_disabled_native/
  (collect_ignore_glob via local conftest.py)
- Sanitizer Layer 7 fix: only collapse horizontal whitespace, preserve
  newlines and tabs so list/table structure survives
- Hardened runtime config.yaml references only the searx-backed tools
- Factory overlay (backend/) kept in sync with deer-flow tree as a
  reference / source

See HARDENING.md for the full audit trail and verification steps.

deer-flow/backend/app/gateway/config.py

@@ -0,0 +1,27 @@
+import os
+
+from pydantic import BaseModel, Field
+
+
+class GatewayConfig(BaseModel):
+    """Configuration for the API Gateway."""
+
+    host: str = Field(default="0.0.0.0", description="Host to bind the gateway server")
+    port: int = Field(default=8001, description="Port to bind the gateway server")
+    cors_origins: list[str] = Field(default_factory=lambda: ["http://localhost:3000"], description="Allowed CORS origins")
+
+
+_gateway_config: GatewayConfig | None = None
+
+
+def get_gateway_config() -> GatewayConfig:
+    """Get gateway config, loading from environment if available."""
+    global _gateway_config
+    if _gateway_config is None:
+        cors_origins_str = os.getenv("CORS_ORIGINS", "http://localhost:3000")
+        _gateway_config = GatewayConfig(
+            host=os.getenv("GATEWAY_HOST", "0.0.0.0"),
+            port=int(os.getenv("GATEWAY_PORT", "8001")),
+            cors_origins=cors_origins_str.split(","),
+        )
+    return _gateway_config