feat: Add Deduplication Agent with embedding and LLM methods

Implement a new Deduplication Agent that identifies and groups similar
transformation descriptions. Supports two deduplication methods:
- Embedding: Fast vector similarity comparison using cosine similarity
- LLM: Accurate pairwise semantic comparison (slower but more precise)

Backend changes:
- Add deduplication router with /deduplicate endpoint
- Add embedding_service for vector-based similarity
- Add llm_deduplication_service for LLM-based comparison
- Improve expert_transformation error handling and progress reporting

Frontend changes:
- Add DeduplicationPanel with interactive group visualization
- Add useDeduplication hook for state management
- Integrate deduplication tab in main App
- Add threshold slider and method selector in sidebar

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/requirements.txt

@@ -4,3 +4,4 @@ httpx>=0.26.0
 pydantic>=2.5.0
 pydantic-settings>=2.1.0
 python-dotenv>=1.0.0
+numpy>=1.26.0