Start LangGraph backend migration foundation

.env.example

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+DATABASE_URL=sqlite:///./test.db
+SECRET_KEY=replace-with-a-random-secret
+DEBUG=false
+
+ZNJJ_ENVIRONMENT=development
+AGENT_BACKEND=fastgpt
+LANGGRAPH_CHECKPOINTER=memory
+
+ANALYSIS_SERVICE_URL=http://127.0.0.1:3000
+ANALYSIS_AUTH_TOKEN=replace-with-local-fastgpt-token
+APP_ID=replace-with-fastgpt-app-id
+
+LLM_API_KEY=replace-with-llm-api-key
+LLM_BASE_URL=https://api.openai.com/v1
+LLM_MODEL=replace-with-model-name
+LLM_TIMEOUT_SECONDS=60
+LLM_MAX_RETRIES=2
+
+VOICE_CONFIG=config/voice-fastgpt-state-xfyunSuperTTS.json