feat: implement background task system with notifications

- Added background task manager with goroutine execution and panic recovery
- Replaced SSE streaming with background task execution for:
  * Price recalculation (RecalculateAll)
  * Stock import (ImportStockLog)
  * Pricelist creation (CreateWithProgress)
- Implemented unified polling for task status and DB connection in frontend
- Added task indicator in top bar showing running tasks count
- Added toast notifications for task completion/error
- Tasks automatically cleaned up after 10 minutes
- Tasks show progress (0-100%) with descriptive messages
- Updated handler constructors to receive task manager
- Added API endpoints for task status (/api/tasks, /api/tasks/:id)

Fixes issue with SSE disconnection on slow connections during long-running operations

web/templates/base.html

@@ -25,6 +25,7 @@
                     </div>
                 </div>
                 <div class="flex items-center space-x-4">
+                    <div id="task-indicator" class="hidden items-center gap-1 text-sm"></div>
                     <span id="db-connection-status" class="inline-flex items-center gap-1 text-xs text-gray-600">
                         <span class="inline-block w-2 h-2 rounded-full bg-gray-400"></span>
                         DB: checking
@@ -102,22 +103,79 @@
             el.title = errorText || 'Database not connected';
         }
 
-        async function refreshDBConnectionStatus() {
+        const knownTasks = new Map(); // taskID -> {status, notified}
+
+        async function refreshSystemStatus() {
             try {
-                const resp = await fetch('/api/db-status');
-                const data = await resp.json();
-                renderDBStatus(data.connected === true, data.error || '');
+                // Poll DB status
+                const dbResp = await fetch('/api/db-status');
+                const dbData = await dbResp.json();
+                renderDBStatus(dbData.connected === true, dbData.error || '');
+
                 const userEl = document.getElementById('db-user');
-                if (data.connected && data.db_user) {
-                    userEl.textContent = '@' + String(data.db_user);
+                if (dbData.connected && dbData.db_user) {
+                    userEl.textContent = '@' + String(dbData.db_user);
                 } else if (userEl) {
                     userEl.textContent = '';
                 }
+
+                // Poll tasks
+                const tasksResp = await fetch('/api/tasks');
+                const tasksData = await tasksResp.json();
+                updateTaskIndicator(tasksData.tasks || []);
             } catch (e) {
                 renderDBStatus(false, 'Status request failed');
             }
         }
 
+        function updateTaskIndicator(tasks) {
+            const indicator = document.getElementById('task-indicator');
+            if (!indicator) return;
+
+            const runningTasks = tasks.filter(t => t.status === 'running');
+
+            if (runningTasks.length > 0) {
+                indicator.classList.remove('hidden');
+                indicator.classList.add('flex');
+                indicator.innerHTML = `
+                    <svg class="animate-spin h-4 w-4 text-orange-600" fill="none" viewBox="0 0 24 24">
+                        <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
+                        <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
+                    </svg>
+                    <span class="text-xs">${runningTasks.length}</span>
+                `;
+                indicator.title = runningTasks.map(t => t.message).join(', ');
+            } else {
+                indicator.classList.add('hidden');
+                indicator.classList.remove('flex');
+                indicator.innerHTML = '';
+            }
+
+            // Check for newly completed/errored tasks
+            tasks.forEach(task => {
+                const known = knownTasks.get(task.id);
+
+                if (!known) {
+                    knownTasks.set(task.id, {status: task.status, notified: false});
+                } else if (known.status === 'running' && task.status !== 'running' && !known.notified) {
+                    // Task transitioned from running to completed/error
+                    if (task.status === 'completed') {
+                        showToast(task.message || 'Задача завершена', 'success');
+                    } else if (task.status === 'error') {
+                        showToast(task.error || 'Ошибка выполнения задачи', 'error');
+                    }
+                    knownTasks.set(task.id, {status: task.status, notified: true});
+                }
+            });
+
+            // Cleanup old tasks from knownTasks
+            for (const [taskID, info] of knownTasks.entries()) {
+                if (!tasks.find(t => t.id === taskID)) {
+                    knownTasks.delete(taskID);
+                }
+            }
+        }
+
         function closeConnectionSettingsModal() {
             const modal = document.getElementById('connection-settings-modal');
             if (!modal) return;
@@ -217,8 +275,8 @@
                 });
             }
 
-            refreshDBConnectionStatus();
-            setInterval(refreshDBConnectionStatus, 10000);
+            refreshSystemStatus();
+            setInterval(refreshSystemStatus, 5000); // 5 second polling
         });
     </script>
 </body>