feat(issues): S4b backend — REST task issue fields + dashboard open-issues

Closes the REST gap S4b's UI needs (S2 only extended MCP tools):
- routes/tasks.py: create/update accept system_ids (set-semantics) + arose_from_id;
  GET/create/update return the task's associated systems. kind=issue already
  flowed via task_kind. Associations set via services/systems (ACL-checked;
  can_write_note already gated).
- services/dashboard.py: _open_issues section (owner-scoped, ranked like other
  task lists, capped) added to build_dashboard. Dashboard test updated for the
  new key.

Refs plan 825 (S4b, backend half).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_services_dashboard.py

@@ -43,12 +43,14 @@ async def test_build_dashboard_composes_sections():
     with patch.object(dash, "_active_projects", AsyncMock(return_value=["P"])), \
          patch.object(dash, "_recently_completed", AsyncMock(return_value=["done"])), \
          patch.object(dash, "_upcoming_events", AsyncMock(return_value=["evt"])), \
+         patch.object(dash, "_open_issues", AsyncMock(return_value=["iss"])), \
          patch.object(dash, "_week_stats", AsyncMock(return_value={"open_total": 4})):
         out = await dash.build_dashboard(user_id=1)
     assert out == {
         "active_projects": ["P"],
         "recently_completed": ["done"],
         "upcoming_events": ["evt"],
+        "open_issues": ["iss"],
         "week_stats": {"open_total": 4},
     }
 
@@ -59,6 +61,7 @@ async def test_build_dashboard_isolates_failing_section():
     with patch.object(dash, "_active_projects", AsyncMock(side_effect=RuntimeError("db down"))), \
          patch.object(dash, "_recently_completed", AsyncMock(return_value=["done"])), \
          patch.object(dash, "_upcoming_events", AsyncMock(return_value=[])), \
+         patch.object(dash, "_open_issues", AsyncMock(return_value=[])), \
          patch.object(dash, "_week_stats", AsyncMock(return_value={})):
         out = await dash.build_dashboard(user_id=1)
     # failing section degrades to its empty default; others still populate