Aggiunge monitoraggio ops: health check, log scanner, dashboard e notifiche.

Introduce incidenti con dedup, job Sidekiq ogni 3 min, scan log cron, /up/deep,
dashboard /admin/ops e push ntfy; include Sentry opzionale e fix test suite.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/Gemfile

@@ -19,6 +19,8 @@ gem "faraday"
 gem "stripe"
 gem "aws-sdk-s3", "~> 1.0", require: false
 gem "kamal", require: false
+gem "sentry-ruby"
+gem "sentry-rails"
 
 group :development, :test do
   gem "debug", platforms: %i[ mri windows ], require: "debug/prelude"