Admin-only: soft-delete a single expenditure item.

Stamps deleted_at/deleted_by plus removed_from_source_at, so the row leaves every listing and total through the same exclusion the re-import soft-removal uses, while the dedicated columns stop the next re-import's un-soft-delete from resurrecting it (the source may well still carry the row — see _upsert_items) and keep "admin deleted" distinguishable from "gone at source". Reversible via the restore endpoint; the row and its categorisation history stay in the table.

Refund rows linked to this item are unlinked, mirroring the FK's SET-NULL-on-hard-delete semantics — the matcher may re-link them to another candidate on a later import, but never back to a deleted row (it only considers active originals).