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/core/ai_quota_service.py

@@ -115,7 +115,7 @@ async def check_ai_quota(
         select(func.count(AIUsage.id)).where(
             AIUsage.user_id == user_id,
             AIUsage.succeeded == True,  # noqa: E712
-            AIUsage.generation_type.in_(["scaffold", "branch_detail", "chat_message", "chat_generate"]),
+            AIUsage.generation_type.in_(["scaffold", "branch_detail", "chat_message", "chat_generate", "copilot_message", "assistant_message"]),
             AIUsage.created_at >= day_start,
         )
     ) or 0