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>

api/routers/workstreams.py

@@ -1,9 +1,10 @@
 import uuid
 import socket
+import time
 from pathlib import Path
 from typing import Any
 
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, Query, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -19,6 +20,10 @@ from api.schemas.workstream import (
 
 router = APIRouter(prefix="/workstreams", tags=["workstreams"])
 
+_INDEX_CACHE: dict[str, Any] | None = None
+_INDEX_CACHE_AT: float = 0.0
+_INDEX_TTL = 30.0
+
 
 def _repo_path(repo: ManagedRepo) -> Path | None:
     hostname = socket.gethostname()
@@ -92,8 +97,15 @@ async def list_workstreams(
 
 
 @router.get("/workplan-index")
-async def workplan_index(session: AsyncSession = Depends(get_session)) -> dict[str, Any]:
+async def workplan_index(
+    refresh: bool = Query(False, description="Force cache invalidation"),
+    session: AsyncSession = Depends(get_session),
+) -> dict[str, Any]:
     """Map file-backed workstream ids to their local workplan filenames."""
+    global _INDEX_CACHE, _INDEX_CACHE_AT
+    if not refresh and _INDEX_CACHE is not None and (time.monotonic() - _INDEX_CACHE_AT) < _INDEX_TTL:
+        return _INDEX_CACHE
+
     result = await session.execute(
         select(ManagedRepo).where(ManagedRepo.status == "active").order_by(ManagedRepo.slug)
     )
@@ -119,7 +131,9 @@ async def workplan_index(session: AsyncSession = Depends(get_session)) -> dict[s
                     "repo_slug": repo.slug,
                     "archived": archived,
                 }
-    return {"workstreams": index}
+    _INDEX_CACHE = {"workstreams": index}
+    _INDEX_CACHE_AT = time.monotonic()
+    return _INDEX_CACHE
 
 
 @router.post("/", response_model=WorkstreamRead, status_code=status.HTTP_201_CREATED)