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

frontend/src/views/admin/UsageView.vue

@@ -495,7 +495,7 @@ const exportToExcel = async () => {
         log.cache_read_cost?.toFixed(6) || '0.000000', log.cache_creation_cost?.toFixed(6) || '0.000000',
         log.rate_multiplier?.toPrecision(4) || '1.00', (log.account_rate_multiplier ?? 1).toPrecision(4),
         log.total_cost?.toFixed(6) || '0.000000', log.actual_cost?.toFixed(6) || '0.000000',
-        (log.total_cost * (log.account_rate_multiplier ?? 1)).toFixed(6), log.first_token_ms ?? '', log.duration_ms,
+        ((log.account_stats_cost ?? log.total_cost) * (log.account_rate_multiplier ?? 1)).toFixed(6), log.first_token_ms ?? '', log.duration_ms,
         log.request_id || '', log.user_agent || '', log.ip_address || ''
       ])
       if (rows.length) {