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/sub/controller.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+
 	"github.com/mhsanaei/3x-ui/v3/internal/logger"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
 )
@@ -417,7 +418,7 @@ func (a *SUBController) ApplyCommonHeaders(
 	c.Writer.Header().Set("Subscription-Userinfo", header)
 	c.Writer.Header().Set("Profile-Update-Interval", updateInterval)
 
-	//Basics
+	// Basics
 	if profileTitle != "" {
 		c.Writer.Header().Set("Profile-Title", "base64:"+base64.StdEncoding.EncodeToString([]byte(profileTitle)))
 	}
@@ -431,7 +432,7 @@ func (a *SUBController) ApplyCommonHeaders(
 		c.Writer.Header().Set("Announce", "base64:"+base64.StdEncoding.EncodeToString([]byte(profileAnnounce)))
 	}
 
-	//Advanced (Happ)
+	// Advanced (Happ)
 	c.Writer.Header().Set("Routing-Enable", strconv.FormatBool(profileEnableRouting))
 	if profileRoutingRules != "" {
 		c.Writer.Header().Set("Routing", profileRoutingRules)