Implement staged history loading with SSE push

- Stage 1: Fetch 100 records immediately for fast display
- Stage 2+: Background fetch up to 1000 records in batches of 100
- Date-based cursor pagination to avoid race conditions
- Deduplication by record ID to prevent duplicates
- SSE push to clients when history cache is updated
- Shared background fetch state for concurrent user requests

server/routes/dashboard.js

@@ -10,6 +10,7 @@ const downloadClientRegistry = require('../utils/downloadClients');
 const sanitizeError = require('../utils/sanitizeError');
 const TagMatcher = require('../services/TagMatcher');
 const { buildUserDownloads } = require('../services/DownloadBuilder');
+const { onHistoryUpdate, offHistoryUpdate } = require('../utils/historyFetcher');
 
 
 // Track active SSE clients for disconnect cleanup
@@ -202,6 +203,17 @@ router.get('/stream', requireAuth, async (req, res) => {
   // Subscribe to poll-complete notifications
   onPollComplete(sendDownloads);
 
+  // Subscribe to history update notifications
+  function sendHistoryUpdate(type) {
+    try {
+      res.write(`event: history-update\ndata: ${JSON.stringify({ type })}\n\n`);
+      console.log(`[SSE] Sent history update for ${type} to ${user.name}`);
+    } catch (err) {
+      console.error('[SSE] Error sending history update:', err.message);
+    }
+  }
+  onHistoryUpdate(sendHistoryUpdate);
+
   // 25s heartbeat comment to keep the connection alive through proxies/load-balancers
   const heartbeat = setInterval(() => {
     try { res.write(': heartbeat\n\n'); } catch { /* ignore — cleanup below handles it */ }
@@ -211,6 +223,7 @@ router.get('/stream', requireAuth, async (req, res) => {
   req.on('close', () => {
     clearInterval(heartbeat);
     offPollComplete(sendDownloads);
+    offHistoryUpdate(sendHistoryUpdate);
     activeClients.delete(username);
     console.log(`[SSE] Client disconnected: ${user.name}`);
   });