feat(backup): check a backup against what is in place, changing nothing (#5823)

#5823 "Check a backup: compare what it would restore with what is in
place, without restoring".

- steward/backup/check.py: pg_restore writes the backup's rows as COPY
  text with no database involved. The rows in place go out through COPY in
  the same form. Tables are compared on their primary keys, over the columns
  both sides have, and grouped by area.
- Sealed values are opened with each side's own key and compared as
  plaintext digests, so a secret sealed twice still matches. They are never
  shown. A backup whose key can't open its own secrets fails the check.
- What Steward records as it runs (last-seen times, a schedule's last run,
  backup history, scraped UniFi fields) is left out of the comparison:
  contents.ACTIVITY, STATE_SETTINGS and HAND_SET_ONLY.
- Settings → Backups:
  - a Check button on each sent backup, which fetches it from the bucket
    (new s3.get_file);
  - "Check a file" to upload one, with the size limit raised before the
    CSRF check reads the form;
  - a results page.
- Each backup Steward sends checks itself once written, and its history
  row says how that went.
- New CLI command `steward check-backup FILE`. CI's image smoke step checks a
  real backup against its source and expects a match.

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

.forgejo/workflows/ci.yml

@@ -235,6 +235,11 @@ jobs:
           sql -d steward -c "INSERT INTO plugin_metrics (id, source_module, resource_name, metric_name, value, recorded_at) VALUES ('smoke-metric', 'smoke', 'smoke', 'smoke', 1, now())"
           sql -d steward -c "INSERT INTO audit_events (id, timestamp, username, action) VALUES ('smoke-audit', now(), 'smoke', 'smoke.backup')"
           docker exec -u app "$SMOKE-app" steward backup --to /tmp/backups
+          # Checked against the database it was made from, it matches (#5823):
+          # a real pg_restore read, and what a running Steward writes by itself
+          # (last-seen times, backup history) left out of the comparison.
+          docker exec -u app "$SMOKE-app" sh -c 'steward check-backup /tmp/backups/steward-*.tar' | tee /tmp/smoke-check.txt
+          grep -q "Matches what is in place" /tmp/smoke-check.txt
           sql -d steward -c "CREATE DATABASE restored"
           docker exec -u app \
             -e "STEWARD_DATABASE_URL=postgresql+asyncpg://steward:steward@$SMOKE-pg:5432/restored" \