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/spec/models/billing/invoice_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe Billing::Invoice do
       amount_cents: 4000,
       status: "issued"
     )
-    expect(invoice).not_to be_valid
+    expect(invoice.valid?(:issue)).to be(false)
     expect(invoice.errors[:pdf]).to be_present
   end