feat: Add configurable stats_store_interval for stats file saves

Add stats_store_interval config option to control how frequently
the statsstore.json file is written to disk. Default remains 10
seconds for backward compatibility.

This allows reducing disk I/O in production deployments and
can help avoid potential file corruption issues when external
processes read the stats file.

tests/threads/test_stats.py

@@ -72,11 +72,20 @@ class TestAPRSDStatsStoreThread(unittest.TestCase):
 
     def test_init(self):
         """Test APRSDStatsStoreThread initialization."""
-        thread = APRSDStatsStoreThread()
-        self.assertEqual(thread.name, 'StatsStore')
-        self.assertEqual(thread.period, 10)
-        self.assertFalse(thread.daemon)
-        self.assertTrue(hasattr(thread, 'loop_count'))
+        with mock.patch('aprsd.threads.stats.CONF') as mock_conf:
+            mock_conf.stats_store_interval = 10
+            thread = APRSDStatsStoreThread()
+            self.assertEqual(thread.name, 'StatsStore')
+            self.assertEqual(thread.period, 10)
+            self.assertFalse(thread.daemon)
+            self.assertTrue(hasattr(thread, 'loop_count'))
+
+    def test_init_with_custom_interval(self):
+        """Test APRSDStatsStoreThread uses stats_store_interval from config."""
+        with mock.patch('aprsd.threads.stats.CONF') as mock_conf:
+            mock_conf.stats_store_interval = 60
+            thread = APRSDStatsStoreThread()
+            self.assertEqual(thread.period, 60)
 
     def test_loop_with_save(self):
         """Test loop method saves stats every call."""