feat(api): expose import row diagnostics (#1644)

* feat(api): expose import row diagnostics

* fix(api): stabilize import row diagnostics

* fix(api): harden import row diagnostics

* fix(api): number Mint import diagnostics rows

* fix(api): enforce unique import row diagnostics

* fix(api): address import row diagnostics review

db/migrate/20260502000000_add_source_row_number_to_import_rows.rb

@@ -0,0 +1,27 @@
+class AddSourceRowNumberToImportRows < ActiveRecord::Migration[7.2]
+  def up
+    add_column :import_rows, :source_row_number, :integer
+
+    execute <<~SQL
+      WITH numbered AS (
+        SELECT id,
+               ROW_NUMBER() OVER (PARTITION BY import_id ORDER BY created_at, id) AS row_number
+        FROM import_rows
+      )
+      UPDATE import_rows
+      SET source_row_number = numbered.row_number
+      FROM numbered
+      WHERE import_rows.id = numbered.id
+    SQL
+
+    change_column_null :import_rows, :source_row_number, false
+    add_check_constraint :import_rows, "source_row_number > 0", name: "chk_import_rows_source_row_number_positive"
+    add_index :import_rows, [ :import_id, :source_row_number ], unique: true, name: "index_import_rows_on_import_id_and_source_row_number"
+  end
+
+  def down
+    remove_index :import_rows, name: "index_import_rows_on_import_id_and_source_row_number"
+    remove_check_constraint :import_rows, name: "chk_import_rows_source_row_number_positive"
+    remove_column :import_rows, :source_row_number
+  end
+end