refactor: 拆分 middleware/router 为单一职责文件，新增站点设置管理页面

middleware:
- 拆分 auth.go → auth.go + auth_token.go + auth_parser.go + auth_admin.go
- 拆分 csrf.go → csrf.go + csrf_token.go
- 拆分 ratelimit.go → ratelimit.go + ratelimit_core.go + ratelimit_cleanup.go
- 修复 import 未使用/缺失问题

router:
- 拆分 router.go → admin.go + api.go + frontend.go + deps_core.go + deps_extra.go

feat(admin): 站点设置管理页面
- 新增 SSR 页面 /admin/site-settings（仅 Owner）
- 审核开关 Toggle 即时保存
- 通用设置展示
- 侧边栏新增站点设置入口
- 注册 UpdateBoolSetting/GetBoolSetting API 路由

internal/middleware/ratelimit_core.go

@@ -0,0 +1,81 @@
+package middleware
+
+import (
+	"fmt"
+	"time"
+)
+
+// RateLimitResult 限流检查结果
+type RateLimitResult struct {
+	Blocked    bool
+	RetryAfter int
+	Message    string
+}
+
+const (
+	accountWindow   = 1 * time.Minute
+	accountMaxFails = 3
+	accountBlockDur = 1 * time.Minute
+	ipWindow        = 1 * time.Minute
+	ipMaxFails      = 10
+	ipBlockDur      = 1 * time.Minute
+	cleanupInterval = 2 * time.Minute
+	maxEntries      = 10000
+)
+
+// windowState 单个维度的限流状态
+type windowState struct {
+	count        int
+	windowStart  time.Time
+	blockedUntil time.Time
+}
+
+// check 核心检查逻辑
+func (rl *RateLimiter) check(m map[string]*windowState, key string, window, blockDur time.Duration, maxFails int, lowKey bool) (RateLimitResult, func()) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := time.Now()
+	state, exists := m[key]
+	if !exists {
+		state = &windowState{}
+		if len(m) < maxEntries {
+			m[key] = state
+		}
+	}
+
+	if !state.blockedUntil.IsZero() && now.Before(state.blockedUntil) {
+		retry := int(state.blockedUntil.Sub(now).Seconds()) + 1
+		msg := "请求过于频繁，请稍后重试"
+		if lowKey {
+			msg = fmt.Sprintf("该账号登录尝试过于频繁，请 %d 秒后重试", retry)
+		}
+		return RateLimitResult{Blocked: true, RetryAfter: retry, Message: msg}, nil
+	}
+
+	if now.Sub(state.windowStart) > window {
+		state.count = 0
+		state.windowStart = now
+		state.blockedUntil = time.Time{}
+	}
+
+	recordFail := func() {
+		rl.mu.Lock()
+		defer rl.mu.Unlock()
+		s := m[key]
+		if s == nil {
+			return
+		}
+		if now.Sub(s.windowStart) > window {
+			s.count = 1
+			s.windowStart = now
+			return
+		}
+		s.count++
+		if s.count >= maxFails {
+			s.blockedUntil = now.Add(blockDur)
+		}
+	}
+
+	return RateLimitResult{Blocked: false}, recordFail
+}