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>

package.json

@@ -74,6 +74,7 @@
     "openai": "^6.16.0",
     "papaparse": "^5.4.1",
     "react": "^19.0.0",
+    "react-day-picker": "^9.13.0",
     "react-dom": "^19.0.0",
     "react-easy-crop": "^5.5.6",
     "react-hook-form": "^7.54.2",