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>

src/scribe/services/dashboard.py

@@ -22,6 +22,7 @@ logger = logging.getLogger(__name__)
 N_PROJECTS = 3        # most-recently-active projects shown
 TASKS_PER_GROUP = 5   # open-task cap per milestone / no-milestone group
 RECENT_DONE_LIMIT = 8 # recently-completed tasks shown
+OPEN_ISSUES_LIMIT = 10 # open issues shown on the dashboard
 WINDOW_DAYS = 7       # look-back (done) / look-ahead (events) window
 _OPEN = ["todo", "in_progress"]
 
@@ -49,11 +50,42 @@ async def _safe(coro, empty):
         return empty
 
 
+async def _open_issues(user_id: int) -> list[dict]:
+    """Open issues (task_kind='issue', not done/cancelled) across the owner's
+    projects, ranked like the other task lists. Owner-scoped, matching the rest
+    of the dashboard."""
+    async with async_session() as session:
+        rows = await session.execute(
+            select(
+                Note.id, Note.title, Note.status, Note.priority,
+                Note.project_id, Project.title,
+            )
+            .join(Project, Project.id == Note.project_id, isouter=True)
+            .where(
+                Note.user_id == user_id,
+                Note.task_kind == "issue",
+                Note.status.in_(_OPEN),
+                Note.deleted_at.is_(None),
+            )
+            .order_by(*_open_order())
+            .limit(OPEN_ISSUES_LIMIT)
+        )
+        return [
+            {
+                "id": iid, "title": title, "status": status,
+                "priority": priority or "none",
+                "project_id": pid, "project_title": pname,
+            }
+            for iid, title, status, priority, pid, pname in rows.fetchall()
+        ]
+
+
 async def build_dashboard(user_id: int) -> dict:
     return {
         "active_projects": await _safe(_active_projects(user_id), []),
         "recently_completed": await _safe(_recently_completed(user_id), []),
         "upcoming_events": await _safe(_upcoming_events(user_id), []),
+        "open_issues": await _safe(_open_issues(user_id), []),
         "week_stats": await _safe(_week_stats(user_id), {}),
     }