Feature: Rechnungs-System (invoices) — Backend komplett

- DB-Migration: invoices + invoice_items Tabellen inkl. Indizes
- routes/invoices.py: vollständiger Admin-Router (prefix /api/admin/invoices)
  - CRUD: Liste, Detail, Erstellen, Senden, Bezahlen, Stornieren
  - PDF-Generierung via fpdf2 mit §14-UStG-Pflichtangaben (Kleinunternehmer-Hinweis)
  - Cashflow-Übersicht und Quartalsbericht
  - PDF-Download-Endpunkt
  - Speicherung in /scaninput + optionaler Paperless-Upload
- mailer.py: send_email() + Backends um optionale PDF-Anhänge erweitert
  (Brevo: base64, SMTP: MIMEApplication)
- main.py: invoices_router registriert
- docker-compose.yml: /volume1/scaninput:/scaninput Volume hinzugefügt

backend/main.py

@@ -253,6 +253,7 @@ from routes.challenges     import router as challenges_router
 from routes.gassi_zeiten   import router as gassi_zeiten_router
 from routes.help           import router as help_router
 from routes.feedback       import router as feedback_router
+from routes.invoices       import router as invoices_router
 
 app.include_router(auth_router,       prefix="/api/auth",       tags=["Auth"])
 app.include_router(dogs_router,       prefix="/api/dogs",       tags=["Hunde"])
@@ -317,6 +318,7 @@ app.include_router(challenges_router,     prefix="/api/challenges",           ta
 app.include_router(gassi_zeiten_router,   prefix="/api/gassi-zeiten",         tags=["Gassi-Zeiten"])
 app.include_router(help_router,           prefix="/api/help",                 tags=["Hilfe/FAQ"])
 app.include_router(feedback_router,       prefix="/api/feedback",              tags=["Feedback"])
+app.include_router(invoices_router)
 
 
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