#3714 Decision log: #3706 deleted-document title backfill was intended and is irreversible

Docs only. Adds docs/decisions/2026-10-21-deleted-document-title-backfill.md, recording the owner's approval of the #3706 backfill (typed yes 14:06 AWST, 21 Oct 2026; recorded 14:42 AWST). The backfill nulls document_title on storage_cleanup_jobs rows for deleted documents. That was intended, and it is irreversible (no down migration).

RAG impact: none, docs only.

npm run check:decisions: 0 errors. The repo-awareness snapshot was refreshed.

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Summary by cubic

Records the owner's approval of the #3706 backfill, which nulls document_title on storage_cleanup_jobs rows for deleted documents.

• The new docs/decisions/2026-10-21-deleted-document-title-backfill.md quotes the owner's specific 15:33 AWST approval naming this backfill, since their blanket "go ahead" reply doesn't satisfy the database-permission rule.
• Notes the backfill is irreversible (no down migration) and that the source migration wins if the two differ.
• Refreshes the repo-awareness snapshot to catalogue the new decision doc.

Summary by CodeRabbit

• Documentation
  • Added a decision record documenting the approved backfill for deleted-document titles.
  • Clarified that deleted documents receive a null title while live-document and public-source reservation data remains unchanged.
  • Recorded that the migration is irreversible, has no rollback migration, and has already been applied.
• Maintenance
  • Updated the repository awareness catalog to include the new decision record.