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/hive-forge/default.nix

@@ -409,6 +409,16 @@ in
     # vhost nor answer DNS for it.
     services.hyperhive.gateway.localNames = lib.optional cfg.behindGateway cfg.domain;
 
+    # This swarm-ui quick-links entry, same `behindGateway` guard as the
+    # vhost/DNS name above — with it off, this host doesn't actually
+    # serve `cfg.domain`, so linking to it would be dead. See
+    # `services.hyperhive.swarm.controller.links`'s description.
+    services.hyperhive.swarm.controller.links = lib.optional cfg.behindGateway {
+      label = "Forge";
+      icon = "⚒";
+      url = "https://${cfg.domain}/";
+    };
+
     # `server_name = forge.domain`, proxies all `/` → forgejo. Tuned for
     # git: `client_max_body_size 1G`, `proxy_read_timeout 1h` (multi-GB
     # clones). SSH stays direct on `forge.sshPort`. See