refactor: 移除 Ops 监控模块

移除未完成的运维监控功能，简化系统架构：
- 删除 ops_handler, ops_service, ops_repo 等后端代码
- 删除 ops 相关数据库迁移文件
- 删除前端 OpsDashboard 页面和 API

backend/internal/server/middleware/ops_auth_error_logger.go

@@ -1,55 +0,0 @@
-package middleware
-
-import (
-	"context"
-	"sync"
-	"time"
-
-	"github.com/Wei-Shaw/sub2api/internal/service"
-)
-
-const (
-	opsAuthErrorLogWorkerCount = 10
-	opsAuthErrorLogQueueSize   = 256
-	opsAuthErrorLogTimeout     = 2 * time.Second
-)
-
-type opsAuthErrorLogJob struct {
-	ops   *service.OpsService
-	entry *service.OpsErrorLog
-}
-
-var (
-	opsAuthErrorLogOnce  sync.Once
-	opsAuthErrorLogQueue chan opsAuthErrorLogJob
-)
-
-func startOpsAuthErrorLogWorkers() {
-	opsAuthErrorLogQueue = make(chan opsAuthErrorLogJob, opsAuthErrorLogQueueSize)
-	for i := 0; i < opsAuthErrorLogWorkerCount; i++ {
-		go func() {
-			for job := range opsAuthErrorLogQueue {
-				if job.ops == nil || job.entry == nil {
-					continue
-				}
-				ctx, cancel := context.WithTimeout(context.Background(), opsAuthErrorLogTimeout)
-				_ = job.ops.RecordError(ctx, job.entry)
-				cancel()
-			}
-		}()
-	}
-}
-
-func enqueueOpsAuthErrorLog(ops *service.OpsService, entry *service.OpsErrorLog) {
-	if ops == nil || entry == nil {
-		return
-	}
-
-	opsAuthErrorLogOnce.Do(startOpsAuthErrorLogWorkers)
-
-	select {
-	case opsAuthErrorLogQueue <- opsAuthErrorLogJob{ops: ops, entry: entry}:
-	default:
-		// Queue is full; drop to avoid blocking request handling.
-	}
-}