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/042_add_pgvector_and_tree_embeddings.py

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+"""Add pgvector extension and tree_embeddings table.
+
+Revision ID: 042
+Revises: 041
+Create Date: 2026-03-04
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers
+revision: str = "042"
+down_revision: str = "041"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Enable pgvector extension
+    op.execute("CREATE EXTENSION IF NOT EXISTS vector")
+
+    op.create_table(
+        "tree_embeddings",
+        sa.Column("id", postgresql.UUID(as_uuid=True), primary_key=True, server_default=sa.text("gen_random_uuid()")),
+        sa.Column("tree_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("trees.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("account_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("accounts.id", ondelete="CASCADE"), nullable=True),
+        sa.Column("chunk_type", sa.String(30), nullable=False),
+        sa.Column("node_type", sa.String(30), nullable=True),
+        sa.Column("node_id", sa.String(100), nullable=True),
+        sa.Column("chunk_text", sa.Text(), nullable=False),
+        sa.Column("embedding_model", sa.String(50), nullable=False, server_default="voyage-3.5"),
+        sa.Column("embedding", sa.Column.__class__, nullable=True),  # placeholder, replaced below
+        sa.Column("meta", postgresql.JSONB(), nullable=False, server_default=sa.text("'{}'::jsonb")),
+        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()")),
+    )
+
+    # Drop the placeholder embedding column and add the vector column
+    op.drop_column("tree_embeddings", "embedding")
+    op.execute("ALTER TABLE tree_embeddings ADD COLUMN embedding vector(1024)")
+
+    # Indexes
+    op.create_index("ix_tree_embeddings_account_id", "tree_embeddings", ["account_id"])
+    op.create_index("ix_tree_embeddings_tree_id", "tree_embeddings", ["tree_id"])
+
+
+def downgrade() -> None:
+    op.drop_table("tree_embeddings")
+    op.execute("DROP EXTENSION IF EXISTS vector")

backend/alembic/versions/043_add_copilot_conversations.py

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+"""Add copilot_conversations table.
+
+Revision ID: 043
+Revises: 042
+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 = "043"
+down_revision: str = "042"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "copilot_conversations",
+        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("session_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("sessions.id", ondelete="SET NULL"), nullable=True),
+        sa.Column("tree_id", postgresql.UUID(as_uuid=True), sa.ForeignKey("trees.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("messages", postgresql.JSONB(), nullable=False, server_default=sa.text("'[]'::jsonb")),
+        sa.Column("current_node_id", sa.String(100), nullable=True),
+        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("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()")),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.text("now()")),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("copilot_conversations")

backend/alembic/versions/044_add_assistant_chats.py

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+"""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")

backend/alembic/versions/045_add_chat_retention_settings.py

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+"""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")