feat(api): dashboard poll optimisation — T1, T2, T3

T1: Cache-Control max-age=60 on /topics/, /repos/, /domains/ list endpoints
    so repeated dashboard polls within a minute are served from browser cache.

T2: ETag middleware (md5 hash) on all JSON GET responses with conditional-GET
    (304 Not Modified) support; If-None-Match and ETag added to CORS headers.
    ETag registered inside CORS so 304s automatically carry CORS headers.

T3: GET /state/deps — lightweight dep-graph endpoint returning open workstreams
    with depends_on/blocks edges only, skipping the 10-table full-summary query.
    Prerequisite for T4 (switching workstreams.md and dependencies.md off /state/summary).

Workplan: CUST-WP-0039-dashboard-poll-optimization.md

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

api/routers/topics.py

@@ -1,6 +1,6 @@
 import uuid
 
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, Response, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -23,9 +23,11 @@ async def _resolve_domain_id(domain_slug: str, session: AsyncSession) -> uuid.UU
 
 @router.get("/", response_model=list[TopicRead])
 async def list_topics(
+    response: Response,
     status: TopicStatus | None = None,
     session: AsyncSession = Depends(get_session),
 ) -> list[Topic]:
+    response.headers["Cache-Control"] = "max-age=60, stale-while-revalidate=30"
     q = select(Topic)
     if status:
         q = q.where(Topic.status == status)