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/main.py

@@ -14,6 +14,7 @@ from app.core.middleware import RequestLoggingMiddleware, ErrorLoggingMiddleware
 from app.core.rate_limit import limiter
 from app.api.router import api_router
 from app.core.scheduler import scheduler, load_all_schedules, _cleanup_expired_ai_conversations
+from app.services.retention_cleanup import cleanup_expired_chats
 from app.core.service_account import ensure_service_account
 
 # Initialize logging configuration
@@ -122,6 +123,15 @@ async def lifespan(app: FastAPI):
         replace_existing=True,
     )
 
+    # Chat retention cleanup (daily)
+    scheduler.add_job(
+        cleanup_expired_chats,
+        trigger="interval",
+        hours=24,
+        id="cleanup_expired_chats",
+        replace_existing=True,
+    )
+
     # Auto-seed trees in background on PR environments
     seed_task = None
     if settings.SEED_ON_DEPLOY: