后端与全仓代码质量清理（golangci 扩展集 · 测试质量 · 并发安全 · 文档同步）

代码质量全量清理，零行为变化：golangci 扩展集 13 类 linter（gosec/modernize/perfsprint/canonicalheader/usestdlibvars/wastedassign/intrange/errorlint/forcetypeassert/recvcheck/exhaustive/unparam）全量修复，测试代码质量（testifylint/thelper/usetesting）25→0，frpc 进程生命周期真 bug（进程组击杀）、全仓 go test -race 6 类数据竞争（含 1 个生产竞争）、SPDX license 头补齐 131 文件、前端测试套件 next-intl 迁移后 44 失败→全绿、过期 swagger 文档重新生成、pnpm-workspace 构建审批。

Experiments: #2-#17, #18, #20, #21, #23
Metric: total_issues 108 → 8 (-92.6%)

internal/apps/upload/util/media.go

@@ -4,6 +4,7 @@
 package util
 
 import (
+	"slices"
 	"strings"
 
 	"github.com/Rain-kl/Wavelet/internal/apps/upload/shared"
@@ -11,32 +12,17 @@ import (
 
 // IsImageExtension reports whether ext is a common image format.
 func IsImageExtension(ext string) bool {
-	for _, imgExt := range []string{"jpg", "jpeg", "png", "webp", "gif"} {
-		if ext == imgExt {
-			return true
-		}
-	}
-	return false
+	return slices.Contains([]string{"jpg", "jpeg", "png", "webp", "gif"}, ext)
 }
 
 // IsArchiveExtension reports whether ext is a common archive format.
 func IsArchiveExtension(ext string) bool {
-	for _, e := range []string{"zip", "rar", "7z", "tar", "gz", "tgz", "bz2", "xz"} {
-		if ext == e {
-			return true
-		}
-	}
-	return false
+	return slices.Contains([]string{"zip", "rar", "7z", "tar", "gz", "tgz", "bz2", "xz"}, ext)
 }
 
 // IsDocumentExtension reports whether ext is a common document format.
 func IsDocumentExtension(ext string) bool {
-	for _, e := range []string{"pdf", "doc", "docx", "xls", "xlsx", "ppt", "pptx", "txt", "md", "csv", "json", "yaml", "yml", "xml"} {
-		if ext == e {
-			return true
-		}
-	}
-	return false
+	return slices.Contains([]string{"pdf", "doc", "docx", "xls", "xlsx", "ppt", "pptx", "txt", "md", "csv", "json", "yaml", "yml", "xml"}, ext)
 }
 
 // NormalizeImageQuality normalizes the requested image quality query parameter.