fix: merge 30 general improvements from release branch

Bug fixes:
- Detached context for GetAccountConcurrencyBatch (prevent all-zero on request cancel)
- Filter soft-deleted users in GetByGroupID
- Stripe CSP policy (allow Stripe.js in script-src and frame-src)
- WebSearch API key validation on save
- RECHARGING status in payment result success check
- Windows test fixes (logger Sync deadlock, config path escaping)

Feature enhancements:
- Webhook multi-instance dispatch (extractOutTradeNo + GetWebhookProvider)
- EasyPay mobile H5 payment (device param + PayURL2)
- SSE error propagation in WebSearch emulation
- AccountStatsCost DTO field for admin usage logs
- Plans sort by sort_order instead of created_at
- UsageMapHook for streaming response usage data
- apicompat Instructions field passthrough
- EffectiveLoadFactor for ops concurrency/metrics
- Usage billing RETURNING balance for notify system
- BulkUpdate mixed channel warning with details
- println to slog migration in auth cache
- Wire ProviderSet cleanup
- CI cache-dependency-path optimization

Frontend:
- Refund eligibility check per provider (canRequestRefund)
- Plan sort_order editing
- Dead code cleanup (simulate_claude_max, client_affinity)
- GroupsView platform switch guard
- channels features_config API type
- UsageView account_stats_cost export

backend/internal/handler/dto/mappers.go

@@ -628,6 +628,7 @@ func UsageLogFromServiceAdmin(l *service.UsageLog) *AdminUsageLog {
 		ModelMappingChain:     l.ModelMappingChain,
 		BillingTier:           l.BillingTier,
 		AccountRateMultiplier: l.AccountRateMultiplier,
+		AccountStatsCost:      l.AccountStatsCost,
 		IPAddress:             l.IPAddress,
 		Account:               AccountSummaryFromService(l.Account),
 	}

backend/internal/handler/dto/types.go

@@ -427,6 +427,8 @@ type AdminUsageLog struct {
 
 	// AccountRateMultiplier 账号计费倍率快照（nil 表示按 1.0 处理）
 	AccountRateMultiplier *float64 `json:"account_rate_multiplier"`
+	// AccountStatsCost 自定义定价规则计算的账号统计费用（nil 表示使用默认公式）
+	AccountStatsCost *float64 `json:"account_stats_cost,omitempty"`
 
 	// IPAddress 用户请求 IP（仅管理员可见）
 	IPAddress *string `json:"ip_address,omitempty"`