feat: replace client polling with Server-Sent Events (SSE)

Server:
- poller.js: add pollSubscribers Set with onPollComplete/offPollComplete;
  notify all SSE callbacks immediately after every successful poll
- dashboard.js: add GET /api/dashboard/stream endpoint (text/event-stream)
  - requireAuth enforced via cookie (no CSRF needed — GET is a safe method)
  - X-Accel-Buffering: no for nginx proxy compatibility
  - 25s heartbeat comments to survive proxy idle timeouts
  - initial payload sent immediately on connect
  - cleanup on req.close: deregister callback, stop heartbeat, remove client
  - active client tracking updated: type='sse', connectedAt, no refreshRateMs

Frontend:
- app.js: replace setInterval/fetchUserDownloads with EventSource
  - startSSE() opens /api/dashboard/stream; stopSSE() closes it
  - first incoming message hides loading spinner
  - showAll toggle re-opens stream with ?showAll=true param
  - logout calls stopSSE() before POST /api/auth/logout
  - status panel: fixed 5s refresh, shows SSE clients + connect duration
  - statusRefreshHandle now always 5s, not tied to old refresh-rate selector
- index.html: remove now-unused refresh-rate <select> element

Docs:
- ARCHITECTURE.md §4.3: update poller description
- ARCHITECTURE.md §5: rename to SSE Stream (§5.2) + Download Matching (§5.3)
- ARCHITECTURE.md §7: update active client tracking description
- ARCHITECTURE.md §9: add /stream endpoint, update /status clients schema
- ARCHITECTURE.md §10: update key functions table; replace Auto-Refresh
  section with Live Push via SSE
- class-server.puml: add /stream to dashboard routes; update ClientInfo
- component.puml: annotate dashboard with SSE note; update label

server/utils/poller.js

@@ -16,6 +16,12 @@ const POLLING_ENABLED = POLL_INTERVAL > 0;
 let polling = false;
 let lastPollTimings = null;
 
+// SSE subscribers: Set of () => void callbacks, each registered by an open /stream connection
+const pollSubscribers = new Set();
+
+function onPollComplete(cb) { pollSubscribers.add(cb); }
+function offPollComplete(cb) { pollSubscribers.delete(cb); }
+
 // Timed fetch helper: runs a fetch and records how long it took
 async function timed(label, fn) {
   const t0 = Date.now();
@@ -184,6 +190,11 @@ async function pollAllServices() {
 
     const elapsed = Date.now() - start;
     console.log(`[Poller] Poll complete in ${elapsed}ms`);
+
+    // Notify all SSE stream connections so they push fresh data immediately
+    for (const cb of pollSubscribers) {
+      try { cb(); } catch { /* subscriber already disconnected */ }
+    }
   } catch (err) {
     console.error(`[Poller] Poll error:`, err.message);
   } finally {
@@ -216,4 +227,4 @@ function getLastPollTimings() {
   return lastPollTimings;
 }
 
-module.exports = { startPoller, stopPoller, pollAllServices, getLastPollTimings, POLL_INTERVAL, POLLING_ENABLED };
+module.exports = { startPoller, stopPoller, pollAllServices, getLastPollTimings, onPollComplete, offPollComplete, POLL_INTERVAL, POLLING_ENABLED };