Add per-product pause/resume checking feature

Users can now pause and resume price checking for individual products
or in bulk via the Actions menu.

Backend:
- Added checking_paused column to products table
- Scheduler skips products with checking_paused=true
- Added POST /products/bulk/pause endpoint for bulk pause/resume

Frontend:
- Added Pause Checking and Resume Checking to bulk Actions menu
- Added filter dropdown (All/Active/Paused) next to sort controls
- Paused products show greyed out with pause icon and "Paused" label
- Progress bar hidden when paused

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/routes/products.ts

@@ -268,4 +268,31 @@ router.delete('/:id', async (req: AuthRequest, res: Response) => {
   }
 });
 
+// Bulk pause/resume checking
+router.post('/bulk/pause', async (req: AuthRequest, res: Response) => {
+  try {
+    const userId = req.userId!;
+    const { ids, paused } = req.body;
+
+    if (!Array.isArray(ids) || ids.length === 0) {
+      res.status(400).json({ error: 'Product IDs array is required' });
+      return;
+    }
+
+    if (typeof paused !== 'boolean') {
+      res.status(400).json({ error: 'Paused status (boolean) is required' });
+      return;
+    }
+
+    const updated = await productQueries.bulkSetCheckingPaused(ids, userId, paused);
+    res.json({
+      message: `${updated} product(s) ${paused ? 'paused' : 'resumed'}`,
+      updated
+    });
+  } catch (error) {
+    console.error('Error bulk updating pause status:', error);
+    res.status(500).json({ error: 'Failed to update pause status' });
+  }
+});
+
 export default router;