swarm-controller: serve swarm-wide service quick links (hyperhive#3289)

New `services.hyperhive.swarm.controller.links` option (listOf {label,
icon, url}, same shape as the per-agent hyperhive.dashboardLinks) plus
a new GET /api/links route serving it, same pattern as the existing
hives/GET /api/hives.

Rather than one central hardcoded list, each service's own module
contributes its own entry when actually enabled on the controller's
host: swarm-authelia.nix, hive-matrix.nix (gated on gui.enable too,
since / on that vhost only serves fluffychat then) and
hive-forge/default.nix (gated on behindGateway) each push one entry,
the same list-merge idiom services.hyperhive.gateway.localNames
already uses. swarm-ui.nix contributes a static entry for its own
same-origin swagger docs. Adding a future service's link is a nix-only
change to that service's own module.

Verified: cargo build/clippy/test -p swarm-controller clean, a
throwaway nixosSystem eval confirms all 4 entries merge correctly into
SWARM_CONTROLLER_LINKS, nix build .#swarm-controller succeeds.

nix/host-modules/swarm-ui.nix

@@ -124,6 +124,19 @@ in
     # makes the name resolve at all.
     services.hyperhive.gateway.localNames = [ cfg.domain ];
 
+    # This UI's own swagger docs, always same-origin (`/api/docs/` below)
+    # so — unlike authelia/matrix/forge's entries — this one needs no
+    # host name and is never conditional on anything but this module
+    # being enabled at all. See
+    # `services.hyperhive.swarm.controller.links`'s description.
+    services.hyperhive.swarm.controller.links = [
+      {
+        label = "API docs";
+        icon = "🧬";
+        url = "/api/docs/";
+      }
+    ];
+
     # The swarm's front page, and the FIRST `auth_request` anywhere in
     # this gateway (everything else is `auth_basic` + htpasswd).
     #