refactor: simplify HA backend peer handling

- parsePeer: use net.SplitHostPort (handles IPv6, stdlib convention)
- tryConnect: take Go strings, own CGO allocation internally
- RegisterToBackend: remove unreachable 'no HA candidates' error path
- Sync Peers field comment between utils and backend config structs

src/frontend/apiserver/utils/config.go

@@ -18,10 +18,10 @@ type BackendConfig struct {
 	From    string   `yaml:"from"`
 	Key     string   `yaml:"key"`
 	// Peers lists additional controller candidates for HA mode in "host:port"
-	// format.  When non-empty, RegisterToBackend iterates all candidates and
-	// connects to whichever responds as primary.  Address+Port remain the
-	// single-host fallback when Peers is empty.
-	Peers   []string `yaml:"peers"`
+	// format.  When non-empty, RegisterToBackend tries each candidate in order
+	// and connects to whichever accepts the join as primary.  Address+Port
+	// serve as a single-host fallback when Peers is empty.
+	Peers []string `yaml:"peers"`
 }
 
 type AppServerConfig struct {