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/mtproto/manager.go

@@ -2,6 +2,7 @@ package mtproto
 
 import (
 	"bufio"
+	"context"
 	"encoding/json"
 	"fmt"
 	"net"
@@ -181,7 +182,7 @@ func (m *Manager) ensureLocked(inst Instance) error {
 			cur.tag = inst.Tag
 			return nil
 		}
-		cur.proc.Stop()
+		_ = cur.proc.Stop()
 		delete(m.procs, inst.Id)
 	}
 	metricsPort, err := FreeLocalPort()
@@ -211,7 +212,7 @@ func (m *Manager) Remove(id int) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if cur, ok := m.procs[id]; ok {
-		cur.proc.Stop()
+		_ = cur.proc.Stop()
 		delete(m.procs, id)
 		_ = os.Remove(configPathForID(id))
 		logger.Infof("mtproto: stopped mtg for inbound %d", id)
@@ -231,7 +232,7 @@ func (m *Manager) Reconcile(desired []Instance) {
 	}
 	for id, cur := range m.procs {
 		if _, ok := want[id]; !ok {
-			cur.proc.Stop()
+			_ = cur.proc.Stop()
 			delete(m.procs, id)
 			_ = os.Remove(configPathForID(id))
 		}
@@ -323,7 +324,7 @@ func (m *Manager) CollectTraffic() []Traffic {
 // for mtg's metrics endpoint and to allocate the per-inbound SOCKS egress
 // bridge port persisted into mtproto inbound settings.
 func FreeLocalPort() (int, error) {
-	l, err := net.Listen("tcp", "127.0.0.1:0")
+	l, err := (&net.ListenConfig{}).Listen(context.Background(), "tcp", "127.0.0.1:0")
 	if err != nil {
 		return 0, err
 	}
@@ -383,7 +384,11 @@ func writeConfig(path string, inst Instance, metricsPort int) error {
 // Best-effort: an unreachable endpoint or unrecognised format yields ok=false.
 func scrapeTraffic(port int) (up int64, down int64, ok bool) {
 	client := http.Client{Timeout: 3 * time.Second}
-	resp, err := client.Get(fmt.Sprintf("http://127.0.0.1:%d/metrics", port))
+	req, reqErr := http.NewRequestWithContext(context.Background(), http.MethodGet, fmt.Sprintf("http://127.0.0.1:%d/metrics", port), nil)
+	if reqErr != nil {
+		return 0, 0, false
+	}
+	resp, err := client.Do(req)
 	if err != nil {
 		return 0, 0, false
 	}
@@ -418,7 +423,7 @@ func scrapeTraffic(port int) (up int64, down int64, ok bool) {
 
 func parseMetricLine(line string) (name string, labels map[string]string, value float64, err error) {
 	labels = map[string]string{}
-	rest := line
+	var rest string
 	if brace := strings.IndexByte(line, '{'); brace >= 0 {
 		name = line[:brace]
 		end := strings.IndexByte(line, '}')

internal/mtproto/process.go

@@ -5,6 +5,7 @@
 package mtproto
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
@@ -154,7 +155,7 @@ func (p *Process) Start() error {
 	if p.IsRunning() {
 		return errors.New("mtg is already running")
 	}
-	cmd := exec.Command(GetBinaryPath(), "run", p.configPath)
+	cmd := exec.CommandContext(context.Background(), GetBinaryPath(), "run", p.configPath)
 	cmd.Stdout = p.logWriter
 	cmd.Stderr = p.logWriter
 	p.cmd = cmd

internal/mtproto/process_windows.go

@@ -7,8 +7,9 @@ import (
 	"sync"
 	"unsafe"
 
-	"github.com/mhsanaei/3x-ui/v3/internal/logger"
 	"golang.org/x/sys/windows"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/logger"
 )
 
 var (
@@ -36,7 +37,7 @@ func ensureKillOnExitJob() (windows.Handle, error) {
 			uint32(unsafe.Sizeof(info)),
 		)
 		if err != nil {
-			windows.CloseHandle(h)
+			_ = windows.CloseHandle(h)
 			killOnExitJobErr = err
 			return
 		}
@@ -59,7 +60,7 @@ func attachChildLifetime(cmd *exec.Cmd) {
 		logger.Warningf("mtproto: OpenProcess for job attach failed: %v", err)
 		return
 	}
-	defer windows.CloseHandle(h)
+	defer func() { _ = windows.CloseHandle(h) }()
 	if err := windows.AssignProcessToJobObject(job, h); err != nil {
 		logger.Warningf("mtproto: AssignProcessToJobObject failed: %v", err)
 	}