Once an endpoint hangs off a shared zone rather than a per-cluster address,
subdomain labels must be unique across the whole zone rather than within one
cluster. Uniqueness was previously advisory -- a pre-read "taken" set with no
database constraint -- so this adds a unique index on the column and makes
the database the arbiter.
CreateAgentNetworkSettings is a plain INSERT that returns the driver error
unwrapped, both of which the allocator depends on: SaveAgentNetworkSettings
is an upsert (which cannot conflict) and wraps failures in a generic internal
error, discarding the message that unique-violation detection needs.
Note for operators: the index is created by a migration that fails, and
therefore blocks startup, on a deployment that already holds two rows with
the same subdomain on different clusters -- which was legal under the old
per-cluster scheme. Audit for duplicates before upgrading.
fabfacee55