refactor: modernise Go code with go fix and manual improvements (#2027)

Apply go fix ./... analysers (Go 1.26) across backend and examples:
- interface{} → any (type alias, no behaviour change)
- for i := 0; i < N; i++ → for range N / for i := range N
- slices.Contains / slices.ContainsFunc replacing manual loops
- strings.SplitSeq replacing strings.Split in range (avoids allocation)
- strings.CutPrefix replacing HasPrefix+TrimPrefix
- min() replacing manual if/else
- fmt.Appendf replacing []byte(fmt.Sprintf(...))
- strings.Builder replacing string += concatenation
- wg.Go(func(){}) replacing wg.Add(1)/go/wg.Done() pattern
- removed redundant ii := i loop variable copies (unnecessary since Go 1.22)

omitempty on struct-typed JSON fields: go fix removed omitempty from
struct-typed fields (time.Time, PostInfo, UserDetailEntry) because
encoding/json's omitempty never applied to struct types — it was always
a no-op. Kept as bare tags (no omitzero replacement) to preserve the
existing serialisation behaviour.

backend/_example/memory_store/server/admin.go

@@ -73,7 +73,7 @@ func (s *RPC) admEnabledHndl(id uint64, params json.RawMessage) (rr jrpc.Respons
 // onEvent returns nothing, callback to OnEvent
 func (s *RPC) admEventHndl(id uint64, params json.RawMessage) (rr jrpc.Response) {
 	var siteID string
-	var ps []interface{}
+	var ps []any
 	if err := json.Unmarshal(params, &ps); err != nil {
 		return jrpc.Response{Error: err.Error()}
 	}

backend/_example/memory_store/server/data_test.go

@@ -217,7 +217,7 @@ func TestRPC_listFlagsHndl(t *testing.T) {
 
 	flags, err = re.ListFlags(verifyFlagReq)
 	require.NoError(t, err)
-	assert.Equal(t, []interface{}{"u1"}, flags)
+	assert.Equal(t, []any{"u1"}, flags)
 	verifiedUsers := make([]string, 0, len(flags))
 	for _, v := range flags {
 		verifiedUsers = append(verifiedUsers, v.(string))

backend/_example/memory_store/server/rpc_test.go

@@ -21,7 +21,7 @@ import (
 )
 
 func chooseRandomUnusedPort() (port int) {
-	for i := 0; i < 10; i++ {
+	for range 10 {
 		port = 40000 + int(rand.Int31n(10000))
 		if ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port)); err == nil {
 			_ = ln.Close()
@@ -34,7 +34,7 @@ func chooseRandomUnusedPort() (port int) {
 func waitForHTTPServerStart(port int) {
 	// wait for up to 3 seconds for server to start before returning it
 	client := http.Client{Timeout: time.Second}
-	for i := 0; i < 300; i++ {
+	for range 300 {
 		time.Sleep(time.Millisecond * 10)
 		if resp, err := client.Get(fmt.Sprintf("http://localhost:%d", port)); err == nil {
 			_ = resp.Body.Close()