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

lib/bingo.rb

@@ -99,6 +99,11 @@ module Bingo
       super
     end
 
+    def before_save
+      self.updated_at = Time.now.utc
+      super
+    end
+
     def operational? = status == 'operational'
     def planned?     = status == 'planned'
 
@@ -126,10 +131,21 @@ module Bingo
     include Localized
     many_to_one :datacenter, class: 'Bingo::Datacenter'
 
+    # A removed incident changes its datacenter's cell.
+    def after_destroy
+      super
+      Datacenter.where(id: datacenter_id).update(updated_at: Time.now.utc)
+    end
+
     def before_create
       self.created_at ||= Time.now.utc
       super
     end
+
+    def before_save
+      self.updated_at = Time.now.utc
+      super
+    end
   end
 
   # Board state for rendering: ordered cells, hits and completed lines.
@@ -145,7 +161,7 @@ module Bingo
       hit   = cells.map { |dc| dc.incidents.any? }
       wins  = LINES.select { |line| line.all? { |i| hit[i] } }
       incidents = cells.flat_map(&:incidents).sort_by { |i| [i.occurred_on, i.id] }.reverse
-      updated = [Datacenter.max(:created_at), Incident.max(:created_at)].compact.max
+      updated = [Datacenter.max(:updated_at), Incident.max(:updated_at)].compact.max
       new(cells, hit, wins, incidents, updated, view)
     end
 
@@ -364,7 +380,10 @@ module Bingo
   end
 
   def self.renumber!(ids = Datacenter.order(:position, :id).select_map(:id))
-    ids.each_with_index { |id, i| Datacenter.where(id: id).update(position: i + 1) }
+    now = Time.now.utc
+    ids.each_with_index do |id, i|
+      Datacenter.where(id: id).exclude(position: i + 1).update(position: i + 1, updated_at: now)
+    end
   end
 
   # "64" → 64.0, "25–30" → 30.0, ">40" → 40.0, "13,7" → 13.7, "" → nil
@@ -388,14 +407,14 @@ module Bingo
     path = File.join($root, 'db', 'bingo_seed.json')
     return unless File.exist?(path)
     seed = JSON.parse(File.read(path, encoding: 'UTF-8'))
-    cols = Datacenter.columns.map(&:to_s) - %w[id created_at]
+    cols = Datacenter.columns.map(&:to_s) - %w[id created_at updated_at]
     DB.connection.transaction do
       ids = seed.fetch('datacenters').each_with_index.to_h do |row, i|
         dc = Datacenter.create(row.slice(*cols).merge('position' => row['position'] || i + 1))
         [dc.slug, dc.id]
       end
       seed.fetch('incidents', []).each do |row|
-        Incident.create(row.slice(*(Incident.columns.map(&:to_s) - %w[id created_at datacenter_id]))
+        Incident.create(row.slice(*(Incident.columns.map(&:to_s) - %w[id created_at updated_at datacenter_id]))
                            .merge('datacenter_id' => ids.fetch(row.fetch('slug'))))
       end
     end