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.

.github/copilot/mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "playwright": {
+      "command": "npx",
+      "args": ["@playwright/mcp@latest", "--browser", "chromium"]
+    }
+  }
+}