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/websearch_config.go

@@ -140,6 +140,16 @@ func (s *SettingService) SaveWebSearchEmulationConfig(ctx context.Context, cfg *
 	}
 	s.mergeExistingAPIKeys(ctx, cfg)
 
+	// After merge, validate all enabled providers have API keys
+	if cfg.Enabled {
+		for _, p := range cfg.Providers {
+			if p.APIKey == "" {
+				return infraerrors.BadRequest("MISSING_API_KEY",
+					fmt.Sprintf("provider %s has no API key configured", p.Type))
+			}
+		}
+	}
+
 	data, err := json.Marshal(cfg)
 	if err != nil {
 		return fmt.Errorf("websearch: marshal config: %w", err)