fix(tls): also cover derived OAuth subdomain in served cert SAN list

#337's first commit added the OAuth-derivation to the DNS interceptor
but missed the served TLS certificate. With a serverURL of
`http://mac.fritz.box:8000` the cert SAN list covered `mac.fritz.box`
but not `macoauth.fritz.box`, so the speaker would resolve the OAuth
host correctly (via the new DNS hijack) and then immediately fail the
TLS handshake — Spotify / Amazon Music token refresh dies before
reaching AfterTouch.

getDomains now calls discovery.DeriveOAuthHostnames(serverURL) and
discovery.DeriveOAuthHostnames(httpsServerURL), feeding the derived
names into the SAN map alongside the existing entries. IP-based
serverURLs continue to produce no derivation (the OAuth construction
is unrecoverable for them — see the existing oauth_target_reachable
health check).

Tests in cmd/soundtouch-service/main_test.go lock in:
  - Hostname serverURL → derived OAuth variant present in SAN list.
  - IP serverURL → no malformed `192oauth.…` entry leaks in.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/soundtouch-service/main.go

@@ -747,6 +747,22 @@ func getDomains(serverURL, httpsServerURL, hostname string, extraHosts []string)
 		domainsMap[strings.ToLower(u.Hostname())] = true
 	}
 
+	// The speaker firmware constructs the OAuth host by appending `oauth`
+	// to the first label of the streaming hostname (see issue #337 and
+	// pkg/discovery/dns.go DeriveOAuthHostnames). The DNS hijack catches
+	// it; the TLS cert must also cover it, otherwise the speaker rejects
+	// the handshake and Spotify / Amazon Music OAuth dies before reaching
+	// AfterTouch. Derive once from each of serverURL and httpsServerURL —
+	// they typically share a hostname but a multi-homed deployment may
+	// differ.
+	for _, h := range discovery.DeriveOAuthHostnames(serverURL) {
+		domainsMap[h] = true
+	}
+
+	for _, h := range discovery.DeriveOAuthHostnames(httpsServerURL) {
+		domainsMap[h] = true
+	}
+
 	// Explicit overrides / additions for multi-homed hosts, reverse proxies,
 	// or browsing the admin UI via a LAN IP that isn't part of serverURL.
 	for _, h := range extraHosts {

cmd/soundtouch-service/main_test.go

@@ -151,3 +151,39 @@ func TestMergeTLSExtraHosts(t *testing.T) {
 		})
 	}
 }
+
+func TestGetDomains_IncludesOAuthDerivation(t *testing.T) {
+	// Hostname-based serverURL: the derived OAuth variant must end up
+	// in the served TLS cert SAN list, otherwise the speaker rejects
+	// the TLS handshake on Spotify / Amazon Music token refresh.
+	got := getDomains("http://mac.fritz.box:8000", "https://mac.fritz.box:8443", "mac.fritz.box", nil)
+
+	want := "macoauth.fritz.box"
+	if !contains(got, want) {
+		t.Errorf("expected SAN list to include %q (derived from serverURL), got: %v", want, got)
+	}
+}
+
+func TestGetDomains_IPServerURLProducesNoOAuthDerivation(t *testing.T) {
+	// IP-based serverURL deliberately yields no derivation (the speaker's
+	// `<first-label>oauth.<rest>` construction would be malformed for an
+	// IP and no DNS resolver can answer for it). The cert SAN list must
+	// not pretend to cover something that can never be queried.
+	got := getDomains("http://192.168.0.30:8000", "https://192.168.0.30:8443", "192.168.0.30", nil)
+
+	for _, h := range got {
+		if h == "192oauth.168.0.30" {
+			t.Errorf("SAN list must not include malformed IP-derived OAuth name, got: %v", got)
+		}
+	}
+}
+
+func contains(haystack []string, needle string) bool {
+	for _, h := range haystack {
+		if h == needle {
+			return true
+		}
+	}
+
+	return false
+}