test: update tests for system settings and user list enhancements

System Settings Tests (new file):
- Create system_settings_test.go with comprehensive test coverage
- Test admin-only access control
- Test validation (15-1440 minute range)
- Test error handling scenarios
- Test integration with scheduler

User Tests Cleanup:
- Remove old TestScanSettings from user_test.go
- Scan settings moved to system-wide (no longer per-user)

Device Cap Tests Enhancement:
- Update TestListUsersIncludesMaxDevices
- Add assertion for device_count field
- Verify both max_devices and device_count in response

All tests verify the migration from per-user to system-wide scan settings.

cmd/server/tests/device_cap_test.go

@@ -288,10 +288,13 @@ func TestListUsersIncludesMaxDevices(t *testing.T) {
 	var users []map[string]interface{}
 	json.NewDecoder(resp.Body).Decode(&users)
 
-	// Verify max_devices field is present in response
+	// Verify max_devices and device_count fields are present in response
 	if len(users) > 0 {
 		_, hasMaxDevices := users[0]["max_devices"]
 		assert.True(t, hasMaxDevices, "max_devices field should be present in user list")
+
+		_, hasDeviceCount := users[0]["device_count"]
+		assert.True(t, hasDeviceCount, "device_count field should be present in user list")
 	}
 }