Update usage metrics for v2026.1: login customization and deleted datasets and properties

[ktuite]

Closes getodk/central#1700

Adds new properties

• under top level system
  • login_customization_logo_set
  • login_customization_hero_image_set
  • login_customization_title_set
  • login_customization_description_set
• per project, under other
  • num_datasets_deleted (count of dataset.delete events grouped by project ID (calculated through dataset actee parent)
  • num_dataset_properties_deleted (count of dataset.update.property.delete events also grouped by project ID)

Steps:

• Updated XLSForm
• Converted it to xml with local version of pyxform and --pretty_print
• Adjusted version in config
• Adjusted fields
• Added queries for checking login configuration and counting deleted datasets
• Added tests

What has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass, or witnessed Github completing all checks with success
• verified that any code from external sources are properly credited in comments or that everything is internally sourced