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/controller/api.go

@@ -5,7 +5,6 @@ import (
 	"strings"
 
 	"github.com/mhsanaei/3x-ui/v3/internal/web/middleware"
-	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service/panel"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service/tgbot"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/session"
@@ -22,7 +21,6 @@ type APIController struct {
 	hostController        *HostController
 	settingController     *SettingController
 	xraySettingController *XraySettingController
-	settingService        service.SettingService
 	userService           panel.UserService
 	apiTokenService       panel.ApiTokenService
 	Tgbot                 tgbot.Tgbot