Changesets: staged deletes of gone records count as done, name stale ops

A web changeset with a staged record delete could not be applied after
the record was deleted elsewhere (e.g. with the CLI), and the error only
said "record N does not exist", so it was hard to find the operation to
remove. A delete of a record that is already gone is now a no-op with a
warning. Other operations that no longer apply fail with their number and
summary (op_summary in the API), and the Changes page marks that row and
offers to remove it from the error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,17 @@ All notable changes to Jarl are documented here. The format follows
 
 ## [Unreleased]
 
+### Fixed
+
+- A changeset with a staged record delete could not be applied after that
+  record was deleted elsewhere (e.g. with the CLI), and it was hard to see
+  which operation to remove: the error only said "record 832 does not
+  exist". A staged delete of a record that is already gone now counts as
+  done (with a warning), and the other operations that no longer apply
+  (an edit or disable of a deleted record, a change to an object changed
+  since) are named with their number and summary. The Changes page marks
+  that operation and offers to remove it right in the error.
+
 ### Added
 
 - Prometheus metrics at `/metrics` ([docs/metrics.md](docs/metrics.md)),