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/accessor/data.go

@@ -251,11 +251,11 @@ func (m *MemData) Flag(req engine.FlagRequest) (val bool, err error) {
 
 // ListFlags get list of flagged keys, like blocked & verified user
 // works for full locator (post flags) or with userID
-func (m *MemData) ListFlags(req engine.FlagRequest) (res []interface{}, err error) {
+func (m *MemData) ListFlags(req engine.FlagRequest) (res []any, err error) {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
 
-	res = []interface{}{}
+	res = []any{}
 
 	switch req.Flag {
 	case engine.Verified:

backend/_example/memory_store/accessor/data_test.go

@@ -198,7 +198,7 @@ func TestMemData_FindForUserPagination(t *testing.T) {
 	}
 
 	// write 200 comments
-	for i := 0; i < 200; i++ {
+	for i := range 200 {
 		c.ID = fmt.Sprintf("idd-%d", i)
 		c.Text = fmt.Sprintf("text #%d", i)
 		c.Timestamp = time.Date(2017, 12, 20, 15, 18, i, 0, time.Local)
@@ -484,7 +484,7 @@ func TestMemData_FlagVerified(t *testing.T) {
 func TestMemData_FlagListVerified(t *testing.T) {
 
 	b := prepMem(t)
-	toIDs := func(inp []interface{}) (res []string) {
+	toIDs := func(inp []any) (res []string) {
 		res = make([]string, len(inp))
 		for i, v := range inp {
 			vv, ok := v.(string)
@@ -530,7 +530,7 @@ func TestMemData_FlagListBlocked(t *testing.T) {
 		return err
 	}
 
-	toBlocked := func(inp []interface{}) (res []store.BlockedUser) {
+	toBlocked := func(inp []any) (res []store.BlockedUser) {
 		res = make([]store.BlockedUser, len(inp))
 		for i, v := range inp {
 			vv, ok := v.(store.BlockedUser)

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()