v0.13.5 - Identity constraints for assets, fields, values and paths

Changes

• Assets: (path, filename) is unique per account, enforced by the database. Uploads from imports and background jobs are no longer rejected because a different account holds the same filename.
• Blueprint fields: handles are unique within a blueprint, and every field must have a handle and a registered type.
• One stored value per field per entry, enforced by the database.
• Content paths: one URL per account and one path per entry. An entry whose new path would collide with an existing one keeps its previous URL and logs the clash.
• collections_save: naming a blueprint_handles entry that another collection already owns is now refused and reported in missing with the owning collection, instead of silently sharing the schema between collections.
• collections_delete, navigations_delete and globals_delete leave a blueprint another owner still claims in place and return the retained handles in the response, rather than failing with an internal error.
• Custom domains: a platform surface (app, api, auth under a platform apex) or another account's subdomain can no longer be claimed as a custom domain.