fix(db): add gateway_id foreign key to agents table

Agents reference gateways but the migration was missing, causing
ensure_main_agent() to fail on INSERT with UndefinedColumn error.

backend/migrations/versions/d2a3b4c5e6f7_add_gateway_id_to_agents.py

@@ -0,0 +1,39 @@
+"""Add gateway_id foreign key to agents table.
+
+Agents need to belong to a gateway so that main-agent provisioning
+and session lookups can resolve which gateway an agent connects to.
+
+Revision ID: d2a3b4c5e6f7
+Revises: c4a1d2e3f4a5
+Create Date: 2026-05-21 04:30:00.000000
+
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision = "d2a3b4c5e6f7"
+down_revision = "c4a1d2e3f4a5"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Add gateway_id column and foreign key to agents."""
+    op.add_column(
+        "agents",
+        sa.Column("gateway_id", sa.Uuid(), nullable=True),
+    )
+    op.create_index("ix_agents_gateway_id", "agents", ["gateway_id"], unique=False)
+    op.create_foreign_key("agents_gateway_id_fkey", "agents", "gateways", ["gateway_id"], ["id"])
+
+
+def downgrade() -> None:
+    """Remove gateway_id from agents."""
+    op.drop_constraint("agents_gateway_id_fkey", "agents", type_="foreignkey")
+    op.drop_index("ix_agents_gateway_id", table_name="agents")
+    op.drop_column("agents", "gateway_id")