feat(health): check speaker <margeURL> against configured hosts

For each device, probe /info and extract the <margeURL> the
speaker is configured to talk to. Compare the hostname against
the service's expected-hosts list (serverURL host +
httpsServerURL host + --tls-extra-host values).

When the speaker is pointed at a host AfterTouch doesn't claim,
emit a warning with two pieces of context:
  - the actual <margeURL>, so the operator sees the drift
  - a copyable `soundtouch-service --tls-extra-host=<host>`
    suggestion, which is the right fix when the speaker should
    keep talking to AfterTouch via the unexpected hostname (the
    other fix is re-migration, which is mentioned in the details).

Reachability / parse failures are intentionally silent here —
speaker_info_reachable already covers those, no need to double-warn.

Required plumbing: Server.SetExpectedHosts so main.go can pass
config.domains in, plus an ExpectedHosts() getter the closure-form
registration reads at run time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/soundtouch-service/main.go

@@ -435,6 +435,7 @@ func main() {
 			sm.GetDNSRunning = server.GetDNSRunning
 			server.SetLogBuffer(logBuf)
 			server.SetHTTPServerURL(config.httpsServerURL)
+			server.SetExpectedHosts(config.domains)
 			server.SetVersionInfo(version, commit, date, repoURL)
 			server.SetDiscoverySettings(config.discoveryInterval, config.discoveryEnabled)
 			server.SetDNSSettings(persisted.DNSEnabled, strings.Join(persisted.DNSUpstream, ","), persisted.DNSBindAddr)