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/app/api/router.py

@@ -8,6 +8,8 @@ from app.api.endpoints import feedback
 from app.api.endpoints import ai_builder
 from app.api.endpoints import ai_fix
 from app.api.endpoints import ai_chat
+from app.api.endpoints import copilot
+from app.api.endpoints import assistant_chat
 
 api_router = APIRouter()
 
@@ -40,3 +42,5 @@ api_router.include_router(feedback.router)
 api_router.include_router(ai_builder.router)
 api_router.include_router(ai_fix.router)
 api_router.include_router(ai_chat.router)
+api_router.include_router(copilot.router)
+api_router.include_router(assistant_chat.router)