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>

tests/conftest.py

@@ -50,6 +50,14 @@ def _schema():
 def _truncate(_schema):
     """Truncate all tables after each test for isolation."""
     from api.models import Base
+    import api.routers.state as _state_router
+    import api.routers.workstreams as _ws_router
+
+    # Reset in-process TTL caches so stale data from a previous test can't bleed through.
+    _state_router._SUMMARY_CACHE = None
+    _state_router._SUMMARY_CACHE_AT = 0.0
+    _ws_router._INDEX_CACHE = None
+    _ws_router._INDEX_CACHE_AT = 0.0
 
     yield
     engine = sqlalchemy.create_engine(_SYNC_URL)