bingo: ping IndexNow when the card changes

A newly recorded incident only showed up in search once a crawler
happened to come back. IndexNow lets Bing, Yandex and the other
participating engines know about changed URLs right away.

Bingo::IndexNow posts the card URLs plus the affected datacenter pages
to api.indexnow.org, in a background thread so admin saves don't wait.
Incidents ping their datacenter. Edits ping the datacenter, or every
page when positions move (create, delete, reorder, sort). A deleted
datacenter's pages are included so the engines see the 404. BingoWeb
serves the key at /<key>.txt.

The key comes from the INDEXNOW Forgejo secret, written to .env as
INDEXNOW_KEY. With no key set this does nothing.

Assisted-by: Claude:opus-5

bingo_web.rb

@@ -188,6 +188,13 @@ class BingoWeb < Sinatra::Base
     erb :dc
   end
 
+  # IndexNow key file: proves to the engines that pings come from us.
+  get %r{/([A-Za-z0-9-]{8,128})\.txt} do |name|
+    halt 404 unless name == Bingo::IndexNow.key
+    content_type 'text/plain'
+    name
+  end
+
   # Icons from assets/bingo/ (public/ is asxp.io's). Search engines want
   # a crawlable favicon URL; a data: URI doesn't count.
   ICONS = { 'favicon.svg' => 'image/svg+xml', 'favicon.ico' => 'image/x-icon',