Fix AI assignment returning nothing: cap tokens, optimize prompt, show errors

- Cap maxTokens at 12000 (was unlimited dynamic calc that could exceed model limits)
- Replace massive EXISTING array with compact CURRENT_JUROR_LOAD counts and
  ALREADY_ASSIGNED per-project map (keeps prompt small across batches)
- Add coverage gap-filler: algorithmically fills projects below required reviews
- Show error state inline on page when AI fails (red banner with message)
- Add server-side logging for debugging assignment flow
- Reduce batch size to 10 projects

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

src/app/(admin)/admin/rounds/[roundId]/page.tsx

@@ -186,7 +186,8 @@ export default function RoundDetailPage() {
       })
     },
     onError: (err) => {
-      toast.error(`AI generation failed: ${err.message}`)
+      toast.error(`AI generation failed: ${err.message}`, { duration: 15000 })
+      console.error('[AI Assignment]', err)
     },
   })
   const [exportOpen, setExportOpen] = useState(false)
@@ -1725,6 +1726,19 @@ export default function RoundDetailPage() {
                     </div>
                   </div>
                 )}
+                {aiAssignmentMutation.error && !aiAssignmentMutation.isPending && (
+                  <div className="flex items-center gap-3 p-3 rounded-lg bg-red-50 border border-red-200 dark:bg-red-950/20 dark:border-red-800">
+                    <AlertTriangle className="h-5 w-5 text-red-600 shrink-0" />
+                    <div className="flex-1">
+                      <p className="text-sm font-medium text-red-800 dark:text-red-200">
+                        AI generation failed
+                      </p>
+                      <p className="text-xs text-red-600 dark:text-red-400">
+                        {aiAssignmentMutation.error.message}
+                      </p>
+                    </div>
+                  </div>
+                )}
                 {aiAssignmentMutation.data && !aiAssignmentMutation.isPending && (
                   <div className="flex items-center gap-3 p-3 rounded-lg bg-emerald-50 border border-emerald-200 dark:bg-emerald-950/20 dark:border-emerald-800">
                     <CheckCircle2 className="h-5 w-5 text-emerald-600 shrink-0" />