feat: Implement resumable background search index rebuild

[wolfyres]

Closes #955.

Introduces a robust, admin-triggered mechanism to rebuild the search index without blocking production read queries.

Acceptance Criteria Met:

[x] Bounded batches & Non-blocking: Processes in batches of 500, yielding to the event loop.

[x] Observable & Resumable: Added /status, /stop, and resume: true parameters utilizing keyset pagination (lastProcessedId).

[x] Dry run: dryRun: true parameter bypasses indexing and queries total document count.

[x] Concurrency rejection: In-memory (or Redis) state lock prevents interleaving of multiple rebuild requests. Returns 409 Conflict.

[x] Runbook documented: Added /docs/runbooks/search-index-rebuild.md with cURL commands and troubleshooting steps.

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[Cedarich]

Kindly fix CI