feat(buchhaltung): recurring job, budget alerts, audit endpoint, konto-typen CRUD

backend/src/database/migrations/081_buchhaltung_alert_threshold.sql

@@ -0,0 +1,8 @@
+-- Migration 081: Add alert_threshold to buchhaltung_konten + default setting
+
+ALTER TABLE buchhaltung_konten
+  ADD COLUMN IF NOT EXISTS alert_threshold INT CHECK (alert_threshold BETWEEN 0 AND 100);
+
+INSERT INTO buchhaltung_einstellungen (key, value)
+VALUES ('default_alert_threshold', '"80"')
+ON CONFLICT (key) DO NOTHING;