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/pkg/logger/stdlog_bridge_test.go

@@ -77,7 +77,7 @@ func TestStdLogBridgeRoutesLevels(t *testing.T) {
 	log.Printf("service started")
 	log.Printf("Warning: queue full")
 	log.Printf("Forward request failed: timeout")
-	Sync()
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
 
 	_ = stdoutW.Close()
 	_ = stderrW.Close()
@@ -139,7 +139,7 @@ func TestLegacyPrintfRoutesLevels(t *testing.T) {
 	LegacyPrintf("service.test", "request started")
 	LegacyPrintf("service.test", "Warning: queue full")
 	LegacyPrintf("service.test", "forward failed: timeout")
-	Sync()
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
 
 	_ = stdoutW.Close()
 	_ = stderrW.Close()