fix: IPv6 URLs, cache-server SIGTERM and atomic task admission (#1217)

Three independent fixes, one commit each.

1. Hosts and ports were interpolated directly when building URLs, so a literal IPv6 address produced an unbracketed authority, making `ACTIONS_CACHE_URL`, `ACTIONS_RUNTIME_URL` and the artifact server listener unusable. Hosts are expected bare, as documented for `cache.host`, so an address that already carries brackets is no longer accepted.
2. `cache-server` waited on its own `os.Interrupt` channel and ignored SIGTERM, so service managers and container runtimes had to kill it.
3. Task admission used a separate `Load` and `Store`, so two concurrent dispatches of the same task id could both be admitted.

Assisted-by: Claude Code:Opus 5
Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1217
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

act/artifactcache/handler.go

@@ -234,7 +234,7 @@ func StartHandler(opts Options) (*Handler, error) {
 
 func (h *Handler) ExternalURL() string {
 	// TODO: make the external url configurable if necessary
-	return fmt.Sprintf("http://%s:%d", h.outboundIP, h.port)
+	return "http://" + net.JoinHostPort(h.outboundIP, strconv.Itoa(h.port))
 }
 
 func (h *Handler) baseURL(cred JobCredential) string {