This is a major upgrade which performs a one-way database migration
and removes LDAP support upstream.

The continuwuity.toml.j2 template is re-synced with the v26.6.1 example
config. Notably, registration_token is now only rendered when non-empty,
because v26+ refuses to start when it is set to an empty string.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>