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/044_add_assistant_chats.py

@@ -0,0 +1,37 @@
+"""Add assistant_chats table.
+
+Revision ID: 044
+Revises: 043
+Create Date: 2026-03-04
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision: str = "044"
+down_revision: str = "043"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "assistant_chats",
+        sa.Column("id", postgresql.UUID(as_uuid=True), primary_key=True, server_default=sa.text("gen_random_uuid()")),
+        sa.Column("user_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True),
+        sa.Column("account_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("accounts.id", ondelete="CASCADE"), nullable=False, index=True),
+        sa.Column("title", sa.String(255), nullable=False, server_default="New Chat"),
+        sa.Column("messages", postgresql.JSONB(), nullable=False, server_default=sa.text("'[]'::jsonb")),
+        sa.Column("message_count", sa.Integer(), nullable=False, server_default=sa.text("0")),
+        sa.Column("total_input_tokens", sa.Integer(), nullable=False, server_default=sa.text("0")),
+        sa.Column("total_output_tokens", sa.Integer(), nullable=False, server_default=sa.text("0")),
+        sa.Column("pinned", sa.Boolean(), nullable=False, server_default=sa.text("false")),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()")),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.text("now()")),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("assistant_chats")