fix(audit): 第二批审计修复 — P0 生产 Bug、安全加固、性能优化、缓存一致性、代码质量

基于 backend-code-audit 审计报告，修复剩余 P0/P1/P2 共 34 项问题：

P0 生产 Bug：
- 修复 time.Since(time.Now()) 计时逻辑错误 (P0-03)
- generateRandomID 改用 crypto/rand 替代固定索引 (P0-04)
- IncrementQuotaUsed 重写为 Ent 原子操作消除 TOCTOU 竞态 (P0-05)

安全加固：
- gateway/openai handler 错误响应替换为泛化消息，防止内部信息泄露 (P1-14)
- usage_log_repo dateFormat 参数改用白名单映射，防止 SQL 注入 (P1-16)
- 默认配置安全加固：sslmode=prefer、response_headers=true、mode=release (P1-18/19, P2-15)

性能优化：
- gateway handler 循环内 defer 替换为显式 releaseWait 闭包 (P1-02)
- group_repo/promo_code_repo Count 前 Clone 查询避免状态污染 (P1-03)
- usage_log_repo 四个查询添加 LIMIT 10000 防止 OOM (P1-07)
- GetBatchUsageStats 添加时间范围参数，默认最近 30 天 (P1-10)
- ip.go CIDR 预编译为包级变量 (P1-11)
- BatchUpdateCredentials 重构为先验证后更新 (P1-13)

缓存一致性：
- billing_cache 添加 jitteredTTL 防止缓存雪崩 (P2-10)
- DeductUserBalance/UpdateSubscriptionUsage 错误传播修复 (P2-12)
- UserService.UpdateBalance 成功后异步失效 billingCache (P2-13)

代码质量：
- search 截断改为按 rune 处理，支持多字节字符 (P2-01)
- TLS Handshake 改为 HandshakeContext 支持 context 取消 (P2-07)
- CORS 预检添加 Access-Control-Max-Age: 86400 (P2-16)

测试覆盖：
- 新增 user_service_test.go（UpdateBalance 缓存失效 6 个用例）
- 新增 batch_update_credentials_test.go（fail-fast + 类型验证 7 个用例）
- 新增 response_transformer_test.go、ip_test.go、usage_log_repo_unit_test.go、search_truncate_test.go
- 集成测试：IncrementQuotaUsed 并发测试、billing_cache 错误传播测试
- config_test.go 补充 server.mode/sslmode 默认值断言

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/pkg/antigravity/response_transformer.go

@@ -1,6 +1,7 @@
 package antigravity
 
 import (
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
 	"log"
@@ -341,12 +342,16 @@ func buildGroundingText(grounding *GeminiGroundingMetadata) string {
 	return builder.String()
 }
 
-// generateRandomID 生成随机 ID
+// generateRandomID 生成密码学安全的随机 ID
 func generateRandomID() string {
 	const chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 	result := make([]byte, 12)
-	for i := range result {
-		result[i] = chars[i%len(chars)]
+	randBytes := make([]byte, 12)
+	if _, err := rand.Read(randBytes); err != nil {
+		panic("crypto/rand unavailable: " + err.Error())
+	}
+	for i, b := range randBytes {
+		result[i] = chars[int(b)%len(chars)]
 	}
 	return string(result)
 }

backend/internal/pkg/antigravity/response_transformer_test.go

@@ -0,0 +1,36 @@
+//go:build unit
+
+package antigravity
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateRandomID_Uniqueness(t *testing.T) {
+	seen := make(map[string]struct{}, 100)
+	for i := 0; i < 100; i++ {
+		id := generateRandomID()
+		require.Len(t, id, 12, "ID 长度应为 12")
+		_, dup := seen[id]
+		require.False(t, dup, "第 %d 次调用生成了重复 ID: %s", i, id)
+		seen[id] = struct{}{}
+	}
+}
+
+func TestGenerateRandomID_Charset(t *testing.T) {
+	const validChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+	validSet := make(map[byte]struct{}, len(validChars))
+	for i := 0; i < len(validChars); i++ {
+		validSet[validChars[i]] = struct{}{}
+	}
+
+	for i := 0; i < 50; i++ {
+		id := generateRandomID()
+		for j := 0; j < len(id); j++ {
+			_, ok := validSet[id[j]]
+			require.True(t, ok, "ID 包含非法字符: %c (ID=%s)", id[j], id)
+		}
+	}
+}

backend/internal/pkg/ip/ip.go

@@ -54,29 +54,34 @@ func normalizeIP(ip string) string {
 	return ip
 }
 
-// isPrivateIP 检查 IP 是否为私有地址。
-func isPrivateIP(ipStr string) bool {
-	ip := net.ParseIP(ipStr)
-	if ip == nil {
-		return false
-	}
+// privateNets 预编译私有 IP CIDR 块，避免每次调用 isPrivateIP 时重复解析
+var privateNets []*net.IPNet
 
-	// 私有 IP 范围
-	privateBlocks := []string{
+func init() {
+	for _, cidr := range []string{
 		"10.0.0.0/8",
 		"172.16.0.0/12",
 		"192.168.0.0/16",
 		"127.0.0.0/8",
 		"::1/128",
 		"fc00::/7",
-	}
-
-	for _, block := range privateBlocks {
-		_, cidr, err := net.ParseCIDR(block)
+	} {
+		_, block, err := net.ParseCIDR(cidr)
 		if err != nil {
-			continue
+			panic("invalid CIDR: " + cidr)
 		}
-		if cidr.Contains(ip) {
+		privateNets = append(privateNets, block)
+	}
+}
+
+// isPrivateIP 检查 IP 是否为私有地址。
+func isPrivateIP(ipStr string) bool {
+	ip := net.ParseIP(ipStr)
+	if ip == nil {
+		return false
+	}
+	for _, block := range privateNets {
+		if block.Contains(ip) {
 			return true
 		}
 	}

backend/internal/pkg/ip/ip_test.go

@@ -0,0 +1,51 @@
+//go:build unit
+
+package ip
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsPrivateIP(t *testing.T) {
+	tests := []struct {
+		name     string
+		ip       string
+		expected bool
+	}{
+		// 私有 IPv4
+		{"10.x 私有地址", "10.0.0.1", true},
+		{"10.x 私有地址段末", "10.255.255.255", true},
+		{"172.16.x 私有地址", "172.16.0.1", true},
+		{"172.31.x 私有地址", "172.31.255.255", true},
+		{"192.168.x 私有地址", "192.168.1.1", true},
+		{"127.0.0.1 本地回环", "127.0.0.1", true},
+		{"127.x 回环段", "127.255.255.255", true},
+
+		// 公网 IPv4
+		{"8.8.8.8 公网 DNS", "8.8.8.8", false},
+		{"1.1.1.1 公网", "1.1.1.1", false},
+		{"172.15.255.255 非私有", "172.15.255.255", false},
+		{"172.32.0.0 非私有", "172.32.0.0", false},
+		{"11.0.0.1 公网", "11.0.0.1", false},
+
+		// IPv6
+		{"::1 IPv6 回环", "::1", true},
+		{"fc00:: IPv6 私有", "fc00::1", true},
+		{"fd00:: IPv6 私有", "fd00::1", true},
+		{"2001:db8::1 IPv6 公网", "2001:db8::1", false},
+
+		// 无效输入
+		{"空字符串", "", false},
+		{"非法字符串", "not-an-ip", false},
+		{"不完整 IP", "192.168", false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := isPrivateIP(tc.ip)
+			require.Equal(t, tc.expected, got, "isPrivateIP(%q)", tc.ip)
+		})
+	}
+}

backend/internal/pkg/tlsfingerprint/dialer.go

@@ -286,7 +286,7 @@ func (d *SOCKS5ProxyDialer) DialTLSContext(ctx context.Context, network, addr st
 		return nil, fmt.Errorf("apply TLS preset: %w", err)
 	}
 
-	if err := tlsConn.Handshake(); err != nil {
+	if err := tlsConn.HandshakeContext(ctx); err != nil {
 		slog.Debug("tls_fingerprint_socks5_handshake_failed", "error", err)
 		_ = conn.Close()
 		return nil, fmt.Errorf("TLS handshake failed: %w", err)