feat: implement Server-Sent Events (SSE) for real-time updates and notifications

docs/API_SSE.md

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+# SSE (Server-Sent Events)
+
+Real-time app state updates via Server-Sent Events. Reduces server load ~95% vs polling.
+
+## Endpoints
+
+| Endpoint | Method | Purpose |
+|----------|--------|---------|
+| `/sse/state` | GET | Stream state updates (requires Bearer token) |
+| `/sse/stats` | GET | Debug: connected clients, queued events |
+
+## Usage
+
+### Connect to SSE Stream
+```bash
+curl -H "Authorization: Bearer YOUR_API_TOKEN" \
+  http://localhost:5000/sse/state
+```
+
+### Check Connection Stats
+```bash
+curl -H "Authorization: Bearer YOUR_API_TOKEN" \
+  http://localhost:5000/sse/stats
+```
+
+## Event Types
+
+- `state_update` - App state changed (e.g., "Scanning", "Processing")
+- `unread_notifications_count_update` - Number of unread notifications changed (count: int)
+
+## Backend Integration
+
+Broadcasts automatically triggered in `app_state.py` via `broadcast_state_update()`:
+
+```python
+from api_server.sse_broadcast import broadcast_state_update
+
+# Called on every state change - no additional code needed
+broadcast_state_update(current_state="Scanning", settings_imported=time.time())
+```
+
+## Frontend Integration
+
+Auto-enabled via `sse_manager.js`:
+
+```javascript
+// In browser console:
+netAlertXStateManager.getStats().then(stats => {
+  console.log("Connected clients:", stats.connected_clients);
+});
+```
+
+## Fallback Behavior
+
+- If SSE fails after 3 attempts, automatically switches to polling
+- Polling starts at 1s, backs off to 30s max
+- No user-visible difference in functionality
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `server/api_server/sse_endpoint.py` | SSE endpoints & event queue |
+| `server/api_server/sse_broadcast.py` | Broadcast helper functions |
+| `front/js/sse_manager.js` | Client-side SSE connection manager |
+
+## Troubleshooting
+
+| Issue | Solution |
+|-------|----------|
+| Connection refused | Check backend running, API token correct |
+| No events received | Verify `broadcast_state_update()` is called on state changes |
+| High memory | Events not processed fast enough, check client logs |
+| Using polling instead of SSE | Normal fallback - check browser console for errors |
+
+---
+
+