OpenReception: Tenant admin self-promotes to GLOBAL_ADMIN

OpenReception's appointment booking software provides an end-to-end encrypted appointment booking platform. Prior to version 1.0.2, a TENANT_ADMIN promotes themselves to platform-wide GLOBAL_ADMIN through a single PUT request. The role-update handler accepts the `GLOBAL_ADMIN` enum value from any tenant admin updating their own tenant's staff. No policy check enforces that "only an existing GLOBAL_ADMIN may grant GLOBAL_ADMIN", so the schema validation IS the authorization decision. After re-login, the JWT contains the new role and the formerly-tenant-scoped admin reaches every other tenant on the platform. On the hosted OpenReception service this is a scope-changed escalation: a single customer-side tenant administrator gains full platform-wide administrative control over all other tenants' configuration, users, staff records, operational metadata, and tenant lifecycle. Plaintext appointment contents remain subject to the E2E model unless chained with the staff-crypto poisoning issue (V-4) or with staff-passkey hijacking (V-1). On a single-tenant self-hosted deployment it is still a privilege escalation because TENANT_ADMIN should not be able to create new tenants, modify global configuration, or manage other administrators. The same handler also accepts updates targeted at any colleague within the tenant. A tenant admin can promote a separate collaborator account instead of themselves, leaving their own audit trail clean while the platform-wide breach happens through a separate identity. Version 1.0.2 fixes the issue.