bingo: lastmod from the last edit, not the last insert

The "updated" date and the sitemap lastmod came from max(created_at),
so editing a datacenter, fixing an incident title or deleting an
incident left them stale. Google stops trusting lastmod once it is
seen to be wrong.

Migration 014 adds updated_at (backfilled from created_at). Saves set
it, renumbering bumps only the rows whose position actually moved, and
deleting an incident bumps its datacenter. The admin delete now goes
through the model so the hook runs. The sitemap gives lastmod as a
full timestamp and drops changefreq, which Google ignores.

Assisted-by: Claude:opus-5

test/bingo_test.rb

@@ -175,6 +175,21 @@ class BingoTest < Minitest::Test
     assert last_response.ok?
     %w[ru en uk].each { |l| assert_includes last_response.body, "<loc>https://example.org/#{l}</loc>" }
     assert_includes last_response.body, 'hreflang="x-default"'
+    refute_includes last_response.body, 'changefreq'
+    assert_match %r{</loc>\n    <lastmod>\d{4}-\d\d-\d\dT\d\d:\d\d:\d\dZ</lastmod>\n    <xhtml:link}, last_response.body
+  end
+
+  def test_updated_on_follows_edits_and_deletes
+    old = Time.utc(2026, 1, 1)
+    DB.connection[:bingo_datacenters].update(created_at: old, updated_at: old)
+    assert_equal old, Bingo::Board.load.updated_on
+    @dcs[5].refresh.update(name_en: 'Renamed')
+    assert_operator Bingo::Board.load.updated_on, :>, old
+    DB.connection[:bingo_datacenters].update(updated_at: old)
+    inc = hit!(2)
+    DB.connection[:bingo_incidents].update(created_at: old, updated_at: old)
+    inc.refresh.destroy
+    assert_operator Bingo::Board.load.updated_on, :>, old
   end
 
   def test_asxpio_static_files_not_served