feat: add AI assistant with in-session copilot and standalone chat with RAG

Implements three-phase AI assistant feature:
- Phase 0: RAG infrastructure with pgvector embeddings, Voyage AI integration,
  tree chunking service, and semantic search over team's flow library
- Phase 1: In-session copilot panel during flow navigation with contextual
  AI help, current step awareness, and suggested related flows
- Phase 2: Standalone AI chat page with persistent conversation history,
  pin/delete, and configurable retention policies (account-level)

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

backend/requirements.txt

@@ -35,6 +35,10 @@ httpx>=0.27.0
 anthropic>=0.40.0
 google-genai>=1.0.0
 
+# RAG / Embeddings
+pgvector>=0.3.6
+voyageai>=0.3.0
+
 # Utilities
 python-dotenv==1.0.1
 croniter>=2.0.0