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

public/index.html

@@ -53,15 +53,6 @@
                         <button class="theme-btn" data-theme="dark">Dark</button>
                         <button class="theme-btn" data-theme="mono">Mono</button>
                     </div>
-                    <div class="refresh-control">
-                        <label for="refresh-rate">Refresh:</label>
-                        <select id="refresh-rate">
-                            <option value="1000">1s</option>
-                            <option value="5000" selected>5s</option>
-                            <option value="10000">10s</option>
-                            <option value="0">Off</option>
-                        </select>
-                    </div>
                     <div id="admin-controls" class="admin-controls" style="display: none;">
                         <label class="toggle-label">
                             <input type="checkbox" id="show-all-toggle">