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/alembic/env.py

@@ -10,6 +10,10 @@ from alembic import context
 # Import your models
 from app.core.database import Base
 from app.models import User, Team, Tree, Session, Attachment, InviteCode
+from app.models.email_verification_token import EmailVerificationToken
+from app.models.tree_embedding import TreeEmbedding
+from app.models.copilot_conversation import CopilotConversation
+from app.models.assistant_chat import AssistantChat
 from app.core.config import settings
 
 # this is the Alembic Config object