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/service/ops_concurrency.go

@@ -64,12 +64,9 @@ func (s *OpsService) getAccountsLoadMapBestEffort(ctx context.Context, accounts
 		if acc.ID <= 0 {
 			continue
 		}
-		c := acc.Concurrency
-		if c <= 0 {
-			c = 1
-		}
-		if prev, ok := unique[acc.ID]; !ok || c > prev {
-			unique[acc.ID] = c
+		lf := acc.EffectiveLoadFactor()
+		if prev, ok := unique[acc.ID]; !ok || lf > prev {
+			unique[acc.ID] = lf
 		}
 	}