fix: rename agent_assignments unique index to match device_id column

The original equipment->device rename migration's ALTER INDEX RENAME was a
no-op (the index didn't exist by the legacy name at that point in the
migration sequence on existing databases), leaving the unique index named
agent_assignments_equipment_id_index. The schema's unique_constraint now
expects agent_assignments_device_id_index, causing a constraint violation
to surface as Ecto.ConstraintError instead of a changeset error.

priv/repo/migrations/20260501183033_rename_agent_assignments_unique_index.exs

@@ -0,0 +1,17 @@
+defmodule Towerops.Repo.Migrations.RenameAgentAssignmentsUniqueIndex do
+  use Ecto.Migration
+
+  def up do
+    execute """
+    ALTER INDEX IF EXISTS agent_assignments_equipment_id_index
+    RENAME TO agent_assignments_device_id_index
+    """
+  end
+
+  def down do
+    execute """
+    ALTER INDEX IF EXISTS agent_assignments_device_id_index
+    RENAME TO agent_assignments_equipment_id_index
+    """
+  end
+end