feat: add receipt upload functionality for Einnahmen and Beleg routes

- Implemented upload and retrieval of receipts (Belege) associated with Einnahmen entries.
- Added new API routes for uploading and deleting receipts.
- Updated the Einnahmen model to include a `belegUrl` field for storing receipt references.
- Enhanced the Einnahmen page to support file uploads and display existing receipts.
- Introduced drag-and-drop functionality for file uploads and improved user feedback during uploads.
- Added necessary validation for file types and sizes during uploads.

prisma/schema.prisma

@@ -115,6 +115,7 @@ model Buchung {
   steuersatz         Int?               // Tax rate: 0, 7, 19 (nullable for non-business records)
   zahlungsart        Zahlungsart?       // KASSE or BANK
   isBusinessRecord   Boolean            @default(false) // True if this is from Einnahme/Ausgabe
+  belegUrl           String?            @db.Text        // Optional receipt/document reference URL
   linkedBuchungId    String?
   linkedBuchung      Buchung?           @relation("BuchungLink", fields: [linkedBuchungId], references: [id], onDelete: SetNull)
   linkedFrom         Buchung[]          @relation("BuchungLink")