feat: add admin ideas management page with private notes

Adds a dedicated super admin page for managing idea submissions across
all tenants. Includes status summary cards, filterable/searchable table,
detail modal with status updates, and private admin notes for internal
tracking (sprint refs, thoughts, follow-ups). Notes are not visible to
tenant users.

- Database: admin_note column on shared.ideas (019 migration)
- Backend: PUT /admin/ideas/:id/note endpoint
- Frontend: AdminIdeasPage with table, filters, detail modal
- Sidebar: "Idea Submissions" nav link in admin sections
- Routing: /admin/ideas route under SuperAdminRoute guard

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

frontend/src/components/layout/Sidebar.tsx

@@ -20,6 +20,7 @@ import {
   IconCalculator,
   IconGitCompare,
   IconScale,
+  IconBulb,
 } from '@tabler/icons-react';
 import { useAuthStore } from '../../stores/authStore';
 
@@ -132,6 +133,13 @@ export function Sidebar({ onNavigate }: SidebarProps) {
           onClick={() => go('/admin')}
           color="red"
         />
+        <NavLink
+          label="Idea Submissions"
+          leftSection={<IconBulb size={18} />}
+          active={location.pathname === '/admin/ideas'}
+          onClick={() => go('/admin/ideas')}
+          color="yellow"
+        />
         {organizations && organizations.length > 0 && (
           <>
             <Divider my="sm" />
@@ -230,6 +238,13 @@ export function Sidebar({ onNavigate }: SidebarProps) {
             onClick={() => go('/admin')}
             color="red"
           />
+          <NavLink
+            label="Idea Submissions"
+            leftSection={<IconBulb size={18} />}
+            active={location.pathname === '/admin/ideas'}
+            onClick={() => go('/admin/ideas')}
+            color="yellow"
+          />
         </>
       )}
     </ScrollArea>