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/models/__init__.py

@@ -29,6 +29,9 @@ from .feedback import Feedback
 from .ai_conversation import AIConversation
 from .ai_usage import AIUsage
 from .ai_chat_session import AIChatSession
+from .tree_embedding import TreeEmbedding
+from .copilot_conversation import CopilotConversation
+from .assistant_chat import AssistantChat
 
 __all__ = [
     "User",
@@ -69,4 +72,7 @@ __all__ = [
     "AIConversation",
     "AIUsage",
     "AIChatSession",
+    "TreeEmbedding",
+    "CopilotConversation",
+    "AssistantChat",
 ]