perf(api): CUST-WP-0041 — DB indexes, TTL caches, noload on list endpoints

- Migration t7o8p9q0r1s2: indexes on tasks.status, tasks(workstream_id,status),
  workstreams.status, sbom_snapshots(repo_id,snapshot_at)
- workplan-index: 30 s TTL cache + ?refresh param (4171 ms → 16 ms on hit)
- /state/summary: 15 s TTL cache, bypassed on Cache-Control: no-cache
- /topics/: noload(workstreams, decisions, progress_events) (2382 ms → 115 ms)
- /domains/: noload(topics, repos, goals) (2252 ms → 39 ms)
- /repos/: noload(goals) (2222 ms → 599 ms first / fast on repeat)
- conftest: reset TTL caches between tests to prevent bleed-through

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

workplans/CUST-WP-0041-api-performance.md

@@ -3,7 +3,7 @@ id: CUST-WP-0041
 type: workplan
 title: "API Performance Optimization"
 domain: custodian
-status: todo
+status: done
 owner: custodian
 topic_slug: custodian
 created: "2026-05-15"