fix(ops): run backups as an admin login, and stop recording failed dumps as good

The Operaciones panel (backup, restore, sync, re-import) shelled out to
mysqldump as the application user, parsed straight out of DATABASE_URL.
`--single-transaction` issues FLUSH TABLES, which needs the global RELOAD
privilege, and the app user is granted only ALL ON jorgecuadros.* plus
USAGE ON *.*. BACKUP failed outright; SYNC and REIMPORT failed with it,
since both take a safety backup first.

An admin credential is now supplied out of band via OPS_DB_ADMIN_USER /
OPS_DB_ADMIN_PASSWORD, mirroring what deploy/scripts/pre-migrate-backup.mjs
already does, rather than permanently elevating the user the API serves
requests as. Host, port and database still come from DATABASE_URL, so the
override can only change who logs in, never which server. Unset, it falls
back to the DATABASE_URL credentials and warns — local development is
unaffected.

Two defects in the dumps themselves, both shared with the deploy backup
before it was rewritten:

- No --set-gtid-purged=OFF. The production server is the replication source
  with GTID on, so every dump embedded SET @@GLOBAL.GTID_PURGED and was
  unrestorable onto the server it came from — the one thing the restore
  screen is for.

- The pipeline's exit status was gzip's, and gzip succeeded. A mysqldump
  that died on its first statement left a small, perfectly valid archive
  that the job recorded as SUCCESS and the restore screen listed as an
  ordinary restore point. Dumps now run under `set -o pipefail`, assert a
  CREATE TABLE count, and delete their own output on failure. Verified with
  a stubbed mysqldump: a failing dump exits 1, surfaces the real error,
  removes the partial file, and — critically — stops SYNC/REIMPORT before
  the ETL touches anything.

Restores gained pipefail too: a corrupt archive made gunzip fail while
mysql, fed a truncated stream, could still exit 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/deploy-galactus.yml

@@ -257,6 +257,8 @@ jobs:
               "DATABASE_URL": "${{ secrets.DATABASE_URL_GALACTUS }}",
               "SESSION_SECRET": "${{ secrets.SESSION_SECRET_GALACTUS }}",
               "SESSION_COOKIE_SECURE": "false",
+              "OPS_DB_ADMIN_USER": "root",
+              "OPS_DB_ADMIN_PASSWORD": "${{ secrets.MYSQL_ROOT_PASSWORD }}",
               "MINIO_ROOT_USER": "${{ secrets.MINIO_ROOT_USER }}",
               "MINIO_ROOT_PASSWORD": "${{ secrets.MINIO_ROOT_PASSWORD }}"
             }