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/endpoints/trees.py

@@ -1,3 +1,4 @@
+import logging
 from datetime import datetime, timezone
 from typing import Annotated, Optional
 from uuid import UUID
@@ -29,6 +30,7 @@ from app.core.audit import log_audit
 from app.core.config import settings
 from app.core.tree_validation import can_publish_tree
 from app.core.step_sync import sync_steps_from_tree, deactivate_synced_steps_for_tree
+from app.services.rag_service import index_tree as rag_index_tree
 
 router = APIRouter(prefix="/trees", tags=["trees"])
 
@@ -542,6 +544,13 @@ async def create_tree(
     )
     tree = result.scalar_one()
 
+    # Index tree for RAG (best-effort, don't fail the request)
+    try:
+        await rag_index_tree(tree.id, db)
+        await db.commit()
+    except Exception:
+        logging.getLogger(__name__).warning("RAG indexing failed for tree %s", tree.id)
+
     return build_full_tree_response(tree)
 
 
@@ -725,6 +734,13 @@ async def update_tree(
     )
     tree = result.scalar_one()
 
+    # Re-index tree for RAG (best-effort)
+    try:
+        await rag_index_tree(tree.id, db)
+        await db.commit()
+    except Exception:
+        logging.getLogger(__name__).warning("RAG re-indexing failed for tree %s", tree.id)
+
     return build_full_tree_response(tree)