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/state.py

@@ -379,6 +379,87 @@ async def _build_domain_summaries(session: AsyncSession) -> list[DomainSummary]:
     ]
 
 
+@router.get("/deps", response_model=list[WorkstreamWithDeps])
+async def get_deps(session: AsyncSession = Depends(get_session)) -> list[WorkstreamWithDeps]:
+    """Lightweight dep-graph endpoint: open workstreams with their dependency edges only.
+
+    Returns the same structure as open_workstreams in /state/summary but skips
+    the 10-table full-summary computation. Task counts are omitted (all zero).
+    Used by workstreams.md and dependencies.md which only need dep edges.
+    """
+    open_ws_rows = await session.execute(
+        select(Workstream)
+        .options(noload("*"))
+        .where(Workstream.status.in_(["active", "blocked"]))
+        .order_by(Workstream.due_date.asc().nullslast(), Workstream.created_at)
+    )
+    open_ws = list(open_ws_rows.scalars().all())
+
+    open_ws_ids = [w.id for w in open_ws]
+    dep_rows = []
+    if open_ws_ids:
+        dep_result = await session.execute(
+            select(WorkstreamDependency).where(
+                (WorkstreamDependency.from_workstream_id.in_(open_ws_ids))
+                | (WorkstreamDependency.to_workstream_id.in_(open_ws_ids))
+            )
+        )
+        dep_rows = list(dep_result.scalars().all())
+
+    dep_ws_ids: set = set()
+    dep_task_ids: set = set()
+    for d in dep_rows:
+        dep_ws_ids.add(d.from_workstream_id)
+        if d.to_workstream_id:
+            dep_ws_ids.add(d.to_workstream_id)
+        if d.to_task_id:
+            dep_task_ids.add(d.to_task_id)
+
+    ws_lookup: dict = {w.id: w for w in open_ws}
+    extra_ids = dep_ws_ids - set(ws_lookup.keys())
+    if extra_ids:
+        extra_rows = await session.execute(
+            select(Workstream).options(noload("*")).where(Workstream.id.in_(extra_ids))
+        )
+        for w in extra_rows.scalars():
+            ws_lookup[w.id] = w
+
+    task_lookup: dict = {}
+    if dep_task_ids:
+        task_rows = await session.execute(select(Task).options(noload("*")).where(Task.id.in_(dep_task_ids)))
+        task_lookup = {t.id: t for t in task_rows.scalars().all()}
+
+    dep_index: dict = {w.id: {"depends_on": [], "blocks": []} for w in open_ws}
+    for d in dep_rows:
+        from_id, to_id, task_id = d.from_workstream_id, d.to_workstream_id, d.to_task_id
+        if from_id in dep_index and to_id and to_id in ws_lookup:
+            dep_index[from_id]["depends_on"].append(WorkstreamDepStub(
+                dep_id=d.id, target_type="workstream", relationship_type=d.relationship_type,
+                workstream_id=to_id, workstream_slug=ws_lookup[to_id].slug,
+                workstream_title=ws_lookup[to_id].title, description=d.description,
+            ))
+        if from_id in dep_index and task_id and task_id in task_lookup:
+            dep_index[from_id]["depends_on"].append(WorkstreamDepStub(
+                dep_id=d.id, target_type="task", relationship_type=d.relationship_type,
+                task_id=task_id, task_title=task_lookup[task_id].title, description=d.description,
+            ))
+        if to_id and to_id in dep_index and from_id in ws_lookup:
+            dep_index[to_id]["blocks"].append(WorkstreamDepStub(
+                dep_id=d.id, target_type="workstream", relationship_type=d.relationship_type,
+                workstream_id=from_id, workstream_slug=ws_lookup[from_id].slug,
+                workstream_title=ws_lookup[from_id].title, description=d.description,
+            ))
+
+    return [
+        WorkstreamWithDeps(
+            **WorkstreamRead.model_validate(w).model_dump(),
+            depends_on=dep_index[w.id]["depends_on"],
+            blocks=dep_index[w.id]["blocks"],
+        )
+        for w in open_ws
+    ]
+
+
 _PRIORITY_RANK = {
     TaskPriority.critical: 0,
     TaskPriority.high: 1,