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/app/api/trpc/[trpc]/route.ts

@@ -3,6 +3,10 @@ import { appRouter } from '@/server/routers/_app'
 import { createContext } from '@/server/context'
 import { checkRateLimit } from '@/lib/rate-limit'
 
+// Allow long-running operations (AI filtering, bulk imports)
+// This affects Next.js serverless functions; for self-hosted, Nginx timeout also matters
+export const maxDuration = 300 // 5 minutes
+
 const RATE_LIMIT = 100 // requests per window
 const RATE_WINDOW_MS = 60 * 1000 // 1 minute