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/config/config_test.go

@@ -233,12 +233,13 @@ func TestLoadForcedCodexInstructionsTemplate(t *testing.T) {
 	configPath := filepath.Join(tempDir, "config.yaml")
 
 	require.NoError(t, os.WriteFile(templatePath, []byte("server-prefix\n\n{{ .ExistingInstructions }}"), 0o644))
-	require.NoError(t, os.WriteFile(configPath, []byte("gateway:\n  forced_codex_instructions_template_file: \""+templatePath+"\"\n"), 0o644))
+	yamlSafePath := filepath.ToSlash(templatePath)
+	require.NoError(t, os.WriteFile(configPath, []byte("gateway:\n  forced_codex_instructions_template_file: \""+yamlSafePath+"\"\n"), 0o644))
 	t.Setenv("DATA_DIR", tempDir)
 
 	cfg, err := Load()
 	require.NoError(t, err)
-	require.Equal(t, templatePath, cfg.Gateway.ForcedCodexInstructionsTemplateFile)
+	require.Equal(t, yamlSafePath, cfg.Gateway.ForcedCodexInstructionsTemplateFile)
 	require.Equal(t, "server-prefix\n\n{{ .ExistingInstructions }}", cfg.Gateway.ForcedCodexInstructionsTemplate)
 }