fix(mining-app): 前台回来时也上传遥测队列（大厂标准做法）

- 进入后台(paused)时立即 flush：session_end + 设备信息不丢失
- 回到前台(resumed)时立即 flush：上传被强杀遗留事件 + 新 session_start

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/mining-app/lib/core/telemetry/session/session_manager.dart

@@ -70,6 +70,8 @@ class SessionManager with WidgetsBindingObserver {
 
     _state = SessionState.foreground;
     _startNewSession();
+    // 上传上次被强杀遗留的事件 + 本次 session_start（含设备信息）
+    _telemetryService?.flushOnBackground();
   }
 
   void _handleBackground() {
@@ -77,6 +79,8 @@ class SessionManager with WidgetsBindingObserver {
 
     _state = SessionState.background;
     _endCurrentSession();
+    // session_end 写入队列后立即上传，不等待阈值
+    _telemetryService?.flushOnBackground();
   }
 
   void _startNewSession() {

frontend/mining-app/lib/core/telemetry/telemetry_service.dart

@@ -290,6 +290,11 @@ class TelemetryService {
     debugPrint('[Telemetry] User ID cleared');
   }
 
+  Future<void> flushOnBackground() async {
+    await _uploader.uploadBatch(batchSize: 50);
+    debugPrint('[Telemetry] Flushed on background');
+  }
+
   Future<void> pauseForLogout() async {
     _uploader.stopPeriodicUpload();
     await _storage.clearEventQueue();