perf: paginate and throttle clearLocal checkpoint deletion

[sleidig]

Summary

Part of #261 (item 7). Based on #262 (e2e suite) — merge that first; GitHub retargets automatically. Independent of the other improvement PRs.

clearLocal (triggered on every permission change and via /admin/clear_local/:db) fetched all _local docs in one buffered response — one replication checkpoint doc exists per client, so large deployments have thousands — and fired every DELETE in parallel via Promise.all. That momentarily floods CouchDB, and a single failure rejected the whole operation while the remaining deletes kept running detached.

Changes

• _local_docs is fetched in pages of 500; since deleted docs disappear from the index, re-fetching naturally pages through the remainder (an attempted-ids set guards against endless loops on undeletable docs).
• Deletes run with bounded concurrency (10 in flight).
• Failures are logged individually, all docs are still attempted, and a summary error is thrown at the end so the admin endpoint still reports failure.

Test plan

• new unit tests: internal-doc skipping, bounded concurrency (max in-flight ≤ 10 across 25 docs), failure handling (all attempted, no endless retry, summary error)
• e2e clear_local tests unchanged and green
• 177 unit / 40 e2e tests green

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between ccdfd25 and bcbb8eb.

📒 Files selected for processing (2)

• src/admin/admin.service.spec.ts
• src/admin/admin.service.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/clear-local-throttling

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