feat: add shadow AI benchmarking for admin model comparison

Add a new admin-only feature that allows the platform owner to benchmark
the production AI model against up to 2 alternate models (any OpenAI-compatible
API) using real tenant data, without impacting users.

Backend:
- Shared AI caller utility (ai-caller.ts) for OpenAI-compatible endpoints
- Shadow AI module with service, controller, and 3 entities
- 6 admin API endpoints for model config CRUD, run trigger, and history
- Auto-creates shadow_ai_models, shadow_runs, shadow_run_results tables
- Exposes health-scores and investment-planning prompt builders for reuse

Frontend:
- New admin page at /admin/shadow-ai with 3 tabs:
  - Model Configuration (production + 2 alternate slots)
  - Run Comparison (tenant select, feature select, side-by-side results)
  - History (filterable run log with detail drill-down)
- Full side-by-side output display with diff highlighting
- Sidebar navigation link for AI Benchmarking

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

frontend/src/components/layout/Sidebar.tsx

@@ -140,6 +140,13 @@ export function Sidebar({ onNavigate }: SidebarProps) {
           onClick={() => go('/admin/ideas')}
           color="yellow"
         />
+        <NavLink
+          label="AI Benchmarking"
+          leftSection={<IconScale size={18} />}
+          active={location.pathname === '/admin/shadow-ai'}
+          onClick={() => go('/admin/shadow-ai')}
+          color="violet"
+        />
         {organizations && organizations.length > 0 && (
           <>
             <Divider my="sm" />
@@ -245,6 +252,13 @@ export function Sidebar({ onNavigate }: SidebarProps) {
             onClick={() => go('/admin/ideas')}
             color="yellow"
           />
+          <NavLink
+            label="AI Benchmarking"
+            leftSection={<IconScale size={18} />}
+            active={location.pathname === '/admin/shadow-ai'}
+            onClick={() => go('/admin/shadow-ai')}
+            color="violet"
+          />
         </>
       )}
     </ScrollArea>