fix(dashboard): infinite scroll, auto-refetch, analytics cache invalidation, remove topic row cap

Frontend:
- Cursor pagination (100/page) replacing hardcoded 80 message limit
- Removed .slice(0,200) cap on mergeMessages
- IntersectionObserver infinite scroll with skeleton loading
- 15s periodic refetch for message list sync

Backend:
- Removed LIMIT 2000 from topic trends SQL query
- Added invalidateAnalyticsCache on message capture (messageCreated)
- Added invalidateAnalyticsCache on batch analysis completion

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/messages/components/MessageFeed.tsx

@@ -1,3 +1,4 @@
+import { useEffect, useRef } from "react";
 import { ScrollArea } from "../../../shared/ui";
 import type { MessageRecord } from "../../../shared/api/client";
 import { MessageCard, MessageCardSkeleton } from "./MessageCard";
@@ -7,9 +8,30 @@ export interface MessageFeedProps {
   onReanalyze: (id: string) => Promise<void>;
   emptyText?: string;
   loading?: boolean;
+  onLoadMore?: () => void;
+  hasMore?: boolean;
+  loadingMore?: boolean;
 }
 
-export function MessageFeed({ messages, onReanalyze, emptyText = "No messages found.", loading }: MessageFeedProps) {
+export function MessageFeed({ messages, onReanalyze, emptyText = "No messages found.", loading, onLoadMore, hasMore, loadingMore }: MessageFeedProps) {
+  // IntersectionObserver for infinite scroll — fires when sentinel becomes visible
+  const sentinelRef = useRef<HTMLDivElement | null>(null);
+
+  useEffect(() => {
+    if (!onLoadMore || !hasMore) return;
+    const el = sentinelRef.current;
+    if (!el) return;
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        if (entries[0]?.isIntersecting) onLoadMore();
+      },
+      { rootMargin: "400px" }, // preload before user reaches bottom
+    );
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [onLoadMore, hasMore]);
+
   if (loading) {
     return (
       <ScrollArea className="h-[calc(100vh-260px)] pr-3">
@@ -30,6 +52,17 @@ export function MessageFeed({ messages, onReanalyze, emptyText = "No messages fo
         {messages.map((message) => (
           <MessageCard key={message.id} message={message} onReanalyze={onReanalyze} />
         ))}
+
+        {/* Infinite-scroll sentinel */}
+        {hasMore && (
+          <div ref={sentinelRef} className="flex items-center justify-center py-4">
+            {loadingMore ? (
+              <MessageCardSkeleton />
+            ) : (
+              <div className="h-2 w-2 rounded-full bg-muted-foreground/40" />
+            )}
+          </div>
+        )}
       </div>
     </ScrollArea>
   );