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/openflare/proxy_route/logics_test.go

@@ -136,7 +136,7 @@ func TestRouteCanMoveAwayFromAlreadyMissingPagesProject(t *testing.T) {
 }
 
 func TestNormalizeCachePolicyDefaultsAndLegacy(t *testing.T) {
-	assert.Equal(t, "", normalizeCachePolicy(false, "static"))
+	assert.Empty(t, normalizeCachePolicy(false, "static"))
 	// Empty/url on write = legacy all (compat); UI sends static explicitly for new default.
 	assert.Equal(t, proxyRouteCachePolicyAll, normalizeCachePolicy(true, ""))
 	assert.Equal(t, proxyRouteCachePolicyStatic, normalizeCachePolicy(true, "static"))
@@ -144,7 +144,7 @@ func TestNormalizeCachePolicyDefaultsAndLegacy(t *testing.T) {
 	assert.Equal(t, proxyRouteCachePolicyAll, normalizeCachePolicy(true, "all"))
 	assert.Equal(t, proxyRouteCachePolicySuffix, normalizeCachePolicy(true, "suffix"))
 
-	assert.Equal(t, "", displayCachePolicy(false, "all"))
+	assert.Empty(t, displayCachePolicy(false, "all"))
 	assert.Equal(t, proxyRouteCachePolicyAll, displayCachePolicy(true, ""))
 	assert.Equal(t, proxyRouteCachePolicyAll, displayCachePolicy(true, "url"))
 	assert.Equal(t, proxyRouteCachePolicyStatic, displayCachePolicy(true, "static"))