fix(core): preserve S3 config and serve nested uploads

[Innei]

Summary

Test plan

• [ ]
• [ ]

Database migrations checklist (delete if no schema change)

• Migration is backwards-compatible with the previous release's code
  (rolling deploy: new and old pods coexist during cutover).
• Drop / rename / type-change is staged across multiple releases
  (expand now, contract in a follow-up release).
• Indexes on large tables use CREATE INDEX CONCURRENTLY (and break
  out of drizzle's default transaction).
• pnpm -C apps/core run lint:migrations passes.
• If migration-lint:allow= is used, reason= explains why and a
  reviewer has agreed.
• Follow-up contract migration tracked in an issue (if applicable).

See .claude/skills/mx-migration-author/SKILL.md for the expand-contract
decision tree and the spec at
docs/superpowers/specs/2026-05-05-database-migration-release-phase-design.md.

[safedep]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the existing region on partial backup patches

When a client PATCHes backupOptions without a region—for example, to change only enable—this transform converts the omitted value from undefined to "auto". patchAndValid() then passes that generated field to patch(), so a previously configured custom S3 region is silently overwritten and subsequent backup uploads may use the wrong region. Keep undefined unchanged while normalizing only an explicitly null or blank region.

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