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>

migrations/versions/t7o8p9q0r1s2_perf_indexes.py

@@ -0,0 +1,27 @@
+"""perf: add missing status and composite indexes
+
+Revision ID: t7o8p9q0r1s2
+Revises: s6n7o8p9q0r1
+Create Date: 2026-05-15
+
+"""
+from alembic import op
+
+revision = "t7o8p9q0r1s2"
+down_revision = "s6n7o8p9q0r1"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_index("ix_tasks_status", "tasks", ["status"])
+    op.create_index("ix_tasks_workstream_status", "tasks", ["workstream_id", "status"])
+    op.create_index("ix_workstreams_status", "workstreams", ["status"])
+    op.create_index("ix_sbom_snapshots_repo_at", "sbom_snapshots", ["repo_id", "snapshot_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_sbom_snapshots_repo_at", table_name="sbom_snapshots")
+    op.drop_index("ix_workstreams_status", table_name="workstreams")
+    op.drop_index("ix_tasks_workstream_status", table_name="tasks")
+    op.drop_index("ix_tasks_status", table_name="tasks")