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/fastgpt_brain.py

@@ -15,7 +15,7 @@ from models import AssistantConfig
 from pipecat.processors.aggregators.llm_context import LLMContext
 from pipecat.processors.frame_processor import FrameProcessor
 
-from services.brains.base import BrainSpec
+from services.brains.base import BaseBrain, BrainSpec
 from services.brains.fastgpt_llm import FastGPTLLMService, normalize_base_url
 
 
@@ -38,15 +38,16 @@ def _extract_welcome(payload: Any) -> str:
     return ""
 
 
-class FastGPTBrain:
+class FastGPTBrain(BaseBrain):
     def __init__(self):
-        self.spec = BrainSpec(
-            type="fastgpt",
-            supported_runtime_modes=frozenset({"pipeline"}),
-            owns_context=False,
-        )
         self._chat_id = uuid4().hex
 
+    spec = BrainSpec(
+        type="fastgpt",
+        supported_runtime_modes=frozenset({"pipeline"}),
+        owns_context=False,
+    )
+
     async def greeting(self, cfg: AssistantConfig) -> str:
         """优先用 FastGPT 后台配置的开场白;无 app_id 或取不到时回退 cfg.greeting。"""
         if not cfg.fastgpt_app_id: