Add Dify integration and enhance workflow node specifications

- Introduce new fields `dify_api_url` and `dify_api_key` in `AssistantConfig` for Dify API integration.
- Update `requirements.txt` to include `dify-client-python` for Dify SDK support.
- Modify `config_resolver` to handle Dify connection information.
- Add a new `globalNode` type in workflow specifications to provide unified settings across workflows.
- Enhance node specifications with additional constraints and default values for better configuration management.
- Update frontend components to support the new `globalNode` type and its properties, improving workflow editor functionality.

backend/services/brains/dify_brain.py

@@ -0,0 +1,61 @@
+"""Dify-hosted brain: prompt, workflow, tools, and context live in Dify."""
+
+from __future__ import annotations
+
+from uuid import uuid4
+
+from dify_client import AsyncClient
+from loguru import logger
+from models import AssistantConfig
+from pipecat.processors.aggregators.llm_context import LLMContext
+from pipecat.processors.frame_processor import FrameProcessor
+
+from services.brains.base import BaseBrain, BrainSpec
+from services.brains.dify_llm import DifyLLMService, normalize_api_base
+
+
+class DifyBrain(BaseBrain):
+    spec = BrainSpec(
+        type="dify",
+        supported_runtime_modes=frozenset({"pipeline"}),
+        owns_context=False,
+    )
+
+    def __init__(self):
+        self._user_id = f"ai-video-{uuid4().hex}"
+        self._client: AsyncClient | None = None
+
+    def _get_client(self, cfg: AssistantConfig) -> AsyncClient:
+        if self._client is None:
+            if not cfg.dify_api_key:
+                raise ValueError("缺少 Dify Agent apiKey")
+            self._client = AsyncClient(
+                api_key=cfg.dify_api_key,
+                api_base=normalize_api_base(cfg.dify_api_url),
+            )
+        return self._client
+
+    async def greeting(self, cfg: AssistantConfig) -> str:
+        """Use Dify's opening statement, with the local greeting as fallback."""
+        try:
+            api_base = normalize_api_base(cfg.dify_api_url)
+            response = await self._get_client(cfg).arequest(
+                f"{api_base}/parameters",
+                "GET",
+                params={"user": self._user_id},
+                timeout=15.0,
+            )
+            opening = str(response.json().get("opening_statement") or "").strip()
+            return opening or cfg.greeting
+        except ValueError:
+            raise
+        except Exception as exc:  # noqa: BLE001 - greeting failure should not block a call
+            logger.warning(f"Dify 获取开场白失败,回退 cfg.greeting: {exc}")
+            return cfg.greeting
+
+    def build_llm(self, cfg: AssistantConfig, context: LLMContext) -> FrameProcessor:
+        return DifyLLMService(
+            cfg,
+            client=self._get_client(cfg),
+            user_id=self._user_id,
+        )