Implement Phase 4~14: LangGraph Agent, RAG pipeline, Gradio Web UI, voice interface

- Upgrade LLM to Qwen3-14B-4bit with Thinking mode (MlxChatModel as LangChain BaseChatModel)
- Add LangGraph ReAct agent with tool calling loop (search_documents, web_search, get_current_date, remember/recall_user_info)
- Add RAG pipeline: BAAI/bge-m3 embeddings + Qdrant vector store + semantic chunking (SemanticSplitter via cosine similarity)
- Replace fixed-size RecursiveCharacterTextSplitter with meaning-based SemanticSplitter (numpy only, no extra deps)
- Add Gradio Web UI (app.py): chat, document ingestion, document management tabs
- Add multi-user support (user_id isolation in DB + per-user agent cache + dropdown selector)
- Add conversation history restore from MySQL on agent init (Phase 11)
- Add UserProfileRepository for persistent user profile (remember/recall tools)
- Add thread-local DB connections to fix pymysql thread-safety with LangGraph ToolNode
- Add Phase 14 voice interface: Whisper STT (microphone → text) + macOS TTS (say -v Yuna)
- Enforce search_documents-first policy in system prompt and tool descriptions
- Update ROADMAP2.md: Phase 14 완료, Phase 13 청킹 부분 완료

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -1,5 +1,5 @@
 # LLM 모델 설정
-MODEL_ID=mlx-community/Qwen2.5-7B-Instruct-4bit
+MODEL_ID=mlx-community/Qwen3-8B-4bit
 MAX_TOKENS=1024
 MAX_HISTORY_TURNS=30
 COMPACT_THRESHOLD=40
@@ -10,3 +10,8 @@ DB_PORT=3306
 DB_NAME=youlbot
 DB_USER=
 DB_PASSWORD=
+
+# LangSmith 트레이싱 (Phase 7) — https://smith.langchain.com 에서 API 키 발급
+LANGCHAIN_TRACING_V2=false
+LANGCHAIN_API_KEY=
+LANGCHAIN_PROJECT=youlbot