fix: deduplicate evidence refs in trend summaries

Backend: assemble_trend_with_evidence now deduplicates document IDs
via dict.fromkeys() (the rollup code already did this, but the base
assembly didn't — same doc could appear multiple times from different
intelligence extractions).

Frontend: Trends.tsx deduplicates via Set before rendering as a safety
net for existing data already stored with duplicates.

frontend/src/pages/Trends.tsx

@@ -103,10 +103,10 @@ function TrendCard({ trend, onClick }: { trend: TrendSummary; onClick: () => voi
       )}
       {expanded && (
         <div className="mt-2 space-y-1 text-xs">
-          {trend.top_supporting_evidence?.map((e, i) => (
+          {[...new Set(trend.top_supporting_evidence ?? [])].map((e, i) => (
             <EvidenceRef key={i} id={e} direction="supporting" />
           ))}
-          {trend.top_opposing_evidence?.map((e, i) => (
+          {[...new Set(trend.top_opposing_evidence ?? [])].map((e, i) => (
             <EvidenceRef key={i} id={e} direction="opposing" />
           ))}
         </div>