`swarm.authelia.url` defaulted to `https://<domain>` only when this host
ran the container, and to `null` otherwise — so the address a client is
given was a statement about co-location rather than about the swarm. A
swarm has one SSO provider; every hive addresses the same name and
resolution decides which address that reaches, exactly as
`swarm.otel.domain` already works.
The option stays nullable: "this swarm has no IdP" is still expressible,
it is just now something an operator states rather than something not
running the container produces. The Grafana fixture that exercised the
no-IdP refusal says it explicitly.
Closes#4536
The single module-eval derivation forced ~62 full nixosSystem
fixtures live at once to compute its cases list: 10.6GB peak RSS /
5m25s to evaluate, by far the dominant cost in nix flake check.
Splits it into 21 independent checks.module-eval-* derivations
(1-7 fixtures each) sharing builders/helpers via module-eval/lib.nix,
so no single derivation needs more than a handful of fixtures live
at once. A few cases spanning two clusters carry a small duplicated
fixture rather than threading shared state through lib.nix.
