test: 完善方案一业务逻辑测试和方案二规模测试

方案一（业务逻辑正确性测试）：
- 修复 SocialAccount.CreatedAt/UpdatedAt NULL 扫描问题（改为 *time.Time 指针）
- 修复 OPLOG_003~006 数据隔离（改用唯一前缀+Search方法隔离）
- 修复 DEV_008 设备列表测试（改用UserID过滤器隔离）
- 修复并发测试 cache=private → cache=shared（SQLite连接共享）
- 新增 testEnv 隔离架构（独立DB + 独立 httptest.Server）

方案二（真实数据规模测试）：
- 新增 LatencyStats P99/P95 百分位统计采集器
- 全部 16 个测试迁移至 newIsolatedDB（独立内存DB，WAL模式）
- 关键查询添加 P99 多次采样统计（UL/LL/DV/DS/PR/AUTH/OPLOG）
- 新增 CONC_SCALE_001~003 并发压测（50-100 goroutine）
- 删除旧 setupScaleTestDB 死代码
- 双阈值体系：SQLite本地宽松阈值 vs PostgreSQL生产严格目标

共计 84 测试通过（68 业务逻辑 + 16 规模测试）

internal/pagination/cursor.go

@@ -0,0 +1,83 @@
+// Package pagination provides cursor-based (keyset) pagination utilities.
+//
+// Unlike offset-based pagination (OFFSET/LIMIT), cursor pagination uses
+// a composite key (typically created_at + id) to locate the "position" in
+// the result set, giving O(limit) performance regardless of how deep you page.
+package pagination
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// Cursor represents an opaque position in a sorted result set.
+// It is serialized as a URL-safe base64 string for transport.
+type Cursor struct {
+	// LastID is the primary key of the last item on the current page.
+	LastID int64 `json:"last_id"`
+	// LastValue is the sort column value of the last item (e.g. created_at).
+	LastValue time.Time `json:"last_value"`
+}
+
+// Encode serializes a Cursor to a URL-safe base64 string suitable for query params.
+func (c *Cursor) Encode() string {
+	if c == nil || c.LastID == 0 {
+		return ""
+	}
+	data, _ := json.Marshal(c)
+	return base64.URLEncoding.WithPadding(base64.NoPadding).EncodeToString(data)
+}
+
+// Decode parses a base64-encoded cursor string back into a Cursor.
+// Returns nil for empty strings (meaning "first page").
+func Decode(encoded string) (*Cursor, error) {
+	if encoded == "" {
+		return nil, nil
+	}
+	data, err := base64.URLEncoding.WithPadding(base64.NoPadding).DecodeString(encoded)
+	if err != nil {
+		return nil, fmt.Errorf("invalid cursor encoding: %w", err)
+	}
+	var c Cursor
+	if err := json.Unmarshal(data, &c); err != nil {
+		return nil, fmt.Errorf("invalid cursor data: %w", err)
+	}
+	return &c, nil
+}
+
+// PageResult wraps a paginated response with cursor navigation info.
+type PageResult[T any] struct {
+	Items      []T    `json:"items"`
+	Total      int64  `json:"total"`       // Approximate or exact total (optional for pure cursor mode)
+	NextCursor string `json:"next_cursor"` // Empty means no more pages
+	HasMore    bool   `json:"has_more"`
+	PageSize   int    `json:"page_size"`
+}
+
+// DefaultPageSize is the default number of items per page.
+const DefaultPageSize = 20
+
+// MaxPageSize caps the maximum allowed items per request to prevent abuse.
+const MaxPageSize = 100
+
+// ClampPageSize ensures size is within [1, MaxPageSize], falling back to DefaultPageSize.
+func ClampPageSize(size int) int {
+	if size <= 0 {
+		return DefaultPageSize
+	}
+	if size > MaxPageSize {
+		return MaxPageSize
+	}
+	return size
+}
+
+// BuildNextCursor creates a cursor from the last item's ID and timestamp.
+// Returns empty string if there are no items.
+func BuildNextCursor(lastID int64, lastTime time.Time) string {
+	if lastID == 0 {
+		return ""
+	}
+	return (&Cursor{LastID: lastID, LastValue: lastTime}).Encode()
+}