Add background filtering jobs, improved date picker, AI reasoning display

- Implement background job system for AI filtering to avoid HTTP timeouts
- Add FilteringJob model to track progress of long-running filtering operations
- Add real-time progress polling for filtering operations on round details page
- Create custom DateTimePicker component with calendar popup (no year picker hassle)
- Fix round date persistence bug (refetchOnWindowFocus was resetting form state)
- Integrate filtering controls into round details page for filtering rounds
- Display AI reasoning for flagged/filtered projects in results table
- Add onboarding system scaffolding (schema, routes, basic UI)
- Allow setting round dates in the past for manual overrides

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/layouts/admin-sidebar.tsx

@@ -91,8 +91,8 @@ const navigation = [
     icon: Handshake,
   },
   {
-    name: 'Forms',
-    href: '/admin/forms' as const,
+    name: 'Onboarding',
+    href: '/admin/onboarding' as const,
     icon: FileText,
   },
 ]