feat: due dates, subtasks, and task detail page (HQ-{number} URLs)

- Schema: added due_date and subtasks JSONB columns to tasks
- API: CRUD endpoints for subtasks (/tasks/:id/subtasks)
- API: due date support in create/update task
- TaskDetailPanel: due date picker with overdue/soon badges
- TaskDetailPanel: subtask checklist with progress bar
- TaskPage: full-page task view at /task/HQ-{number}
- Dashboard: task cards link to detail page, show subtask progress & due date badges
- Migration: 0001_mighty_callisto.sql

backend/drizzle/meta/_journal.json

@@ -8,6 +8,13 @@
       "when": 1769658956087,
       "tag": "0000_grey_starhawk",
       "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "7",
+      "when": 1769670192629,
+      "tag": "0001_mighty_callisto",
+      "breakpoints": true
     }
   ]
 }