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/versions/045_add_chat_retention_settings.py

@@ -0,0 +1,31 @@
+"""Add chat retention settings to accounts.
+
+Revision ID: 045
+Revises: 044
+Create Date: 2026-03-04
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "045"
+down_revision: str = "044"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "accounts",
+        sa.Column("chat_retention_days", sa.Integer(), nullable=True, server_default=sa.text("90")),
+    )
+    op.add_column(
+        "accounts",
+        sa.Column("chat_retention_max_count", sa.Integer(), nullable=True, server_default=sa.text("100")),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("accounts", "chat_retention_max_count")
+    op.drop_column("accounts", "chat_retention_days")