Gutschriften Phase 1: Backend (Modell, Nummernkreis, CRUD, USt-Rechnung)

Neues Feature Gutschriftsverwaltung (Subventionen am Vertrag):

- Modelle CreditNote + CreditNoteNumberRange + Enums (GELD/SACHWERT,
  PRIVAT/FIRMA, NETTO/BRUTTO) + Migration (IF NOT EXISTS, auf Dev
  angewandt).
- USt pro Gutschrift waehlbar (vatRelevant + Basis Netto/Brutto +
  Satz); Netto/USt/Brutto werden berechnet und getrennt gespeichert
  (ZUGFeRD-tauglich). Kundentyp Privat/Firma aus Kunde vorbelegt.
- Nummernkreis in Settings verwaltbar; Nummernvergabe transaktional
  mit SELECT ... FOR UPDATE (keine Doppelvergabe). Bsp GS-2026-0001.
- Service/Controller/Routes: GET/POST /contracts/:id/credit-notes,
  GET .../defaults, GET/PUT/DELETE /credit-notes/:id,
  GET/PUT /credit-notes/number-range. Portal-Token geblockt (interner
  Bereich), CREATE/UPDATE/DELETE auditiert.

Verifiziert: USt-Rechnung (200 netto->238, 200 brutto->168,07+31,93)
und fortlaufende Nummernvergabe.

Phase 2 (Vertrag-UI + Beleg-Upload + Nummernkreis-UI) und Phase 3
(PDF + ZUGFeRD) folgen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/index.ts

@@ -38,6 +38,7 @@ import documentRoutes from './routes/document.routes.js';
 import meterRoutes from './routes/meter.routes.js';
 import stressfreiEmailRoutes from './routes/stressfreiEmail.routes.js';
 import contractRoutes from './routes/contract.routes.js';
+import creditNoteRoutes from './routes/creditNote.routes.js';
 import platformRoutes from './routes/platform.routes.js';
 import cancellationPeriodRoutes from './routes/cancellation-period.routes.js';
 import contractDurationRoutes from './routes/contract-duration.routes.js';
@@ -355,6 +356,7 @@ app.use('/api/documents', documentRoutes);
 app.use('/api/meters', meterRoutes);
 app.use('/api/stressfrei-emails', stressfreiEmailRoutes);
 app.use('/api/contracts', contractRoutes);
+app.use('/api/credit-notes', creditNoteRoutes);
 app.use('/api/platforms', platformRoutes);
 app.use('/api/cancellation-periods', cancellationPeriodRoutes);
 app.use('/api/contract-durations', contractDurationRoutes);