docs: plan scalable rebuild architecture for backup restores

[kphunter]

Summary

• add a decomposition-ready architecture plan for scalable rebuild-tiles execution after restore
• document the supervisor-plus-bounded-workers direction, with explicit state, leasing, cancellation, disk-space safeguards, and rollout phases
• link the new plan from the existing backup/import docs so future implementation work starts from the operator-facing context

Validation

• nix-shell -p nodejs --run 'cd /private/tmp/hriv-issue-863-plan && /Users/kyle/projects/github/hriv/frontend/node_modules/.bin/prettier --check docs/rebuild-tiles-architecture-plan.md docs/admin-import-export.md docs/backup-and-disaster-recovery.md --config .prettierrc.json'

Closes #863

[devin-ai-integration]

Devin Review found 1 potential issue.

[devin-ai-integration]

📝 Info: Missing blank line causes paragraph merge in backup doc

At docs/backup-and-disaster-recovery.md:144-145, the new text ("For the staged plan...") immediately follows the previous sentence ("for how missing vs stale is determined.") without a blank line separator. In Markdown, this causes both sentences to render as a single paragraph. The preceding paragraph at lines 142-144 discusses API references and provenance docs, while the new text at 145-147 introduces the architecture plan — these are logically separate topics. A blank line between them would improve readability and make the architecture plan link visually distinct. This is cosmetic and doesn't affect link functionality.

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