Rejects pending expense entries by voiding them

Instead of deleting pending expense entries, marks them as voided by adding a #voided tag.
This ensures an audit trail while excluding them from balances.

Updates the Fava client to use 'params' for the delete request.

fava_client.py

@@ -652,7 +652,7 @@ class FavaClient:
             async with httpx.AsyncClient(timeout=self.timeout) as client:
                 response = await client.delete(
                     f"{self.base_url}/source_slice",
-                    json={
+                    params={
                         "entry_hash": entry_hash,
                         "sha256sum": sha256sum
                     }