style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

internal/web/service/inbound_node.go

@@ -999,7 +999,7 @@ func (s *InboundService) GetClientsLastOnline() (map[string]int64, error) {
 	db := database.GetDB()
 	var rows []xray.ClientTraffic
 	err := db.Model(&xray.ClientTraffic{}).Select("email, last_online").Find(&rows).Error
-	if err != nil && err != gorm.ErrRecordNotFound {
+	if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
 		return nil, err
 	}
 	result := make(map[string]int64, len(rows))
@@ -1029,7 +1029,7 @@ func (s *InboundService) FilterAndSortClientEmails(emails []string) ([]string, [
 	clients := make([]xray.ClientTraffic, 0, len(uniqEmails))
 	for _, batch := range chunkStrings(uniqEmails, sqliteMaxVars) {
 		var page []xray.ClientTraffic
-		if err := db.Where("email IN ?", batch).Find(&page).Error; err != nil && err != gorm.ErrRecordNotFound {
+		if err := db.Where("email IN ?", batch).Find(&page).Error; err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
 			return nil, nil, err
 		}
 		clients = append(clients, page...)