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/server/routers/_app.ts

@@ -16,6 +16,7 @@ import { partnerRouter } from './partner'
 import { notionImportRouter } from './notion-import'
 import { typeformImportRouter } from './typeform-import'
 import { applicationFormRouter } from './applicationForm'
+import { onboardingRouter } from './onboarding'
 // Phase 2B routers
 import { tagRouter } from './tag'
 import { applicantRouter } from './applicant'
@@ -51,6 +52,7 @@ export const appRouter = router({
   notionImport: notionImportRouter,
   typeformImport: typeformImportRouter,
   applicationForm: applicationFormRouter,
+  onboarding: onboardingRouter,
   // Phase 2B routers
   tag: tagRouter,
   applicant: applicantRouter,