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/apicompat/chatcompletions_to_responses.go

@@ -27,13 +27,14 @@ func ChatCompletionsToResponses(req *ChatCompletionsRequest) (*ResponsesRequest,
 	}
 
 	out := &ResponsesRequest{
-		Model:       req.Model,
-		Input:       inputJSON,
-		Temperature: req.Temperature,
-		TopP:        req.TopP,
-		Stream:      true, // upstream always streams
-		Include:     []string{"reasoning.encrypted_content"},
-		ServiceTier: req.ServiceTier,
+		Model:        req.Model,
+		Instructions: req.Instructions,
+		Input:        inputJSON,
+		Temperature:  req.Temperature,
+		TopP:         req.TopP,
+		Stream:       true, // upstream always streams
+		Include:      []string{"reasoning.encrypted_content"},
+		ServiceTier:  req.ServiceTier,
 	}
 
 	storeFalse := false

backend/internal/pkg/apicompat/types.go

@@ -152,6 +152,7 @@ type AnthropicDelta struct {
 // ResponsesRequest is the request body for POST /v1/responses.
 type ResponsesRequest struct {
 	Model           string              `json:"model"`
+	Instructions    string              `json:"instructions,omitempty"`
 	Input           json.RawMessage     `json:"input"` // string or []ResponsesInputItem
 	MaxOutputTokens *int                `json:"max_output_tokens,omitempty"`
 	Temperature     *float64            `json:"temperature,omitempty"`
@@ -337,6 +338,7 @@ type ResponsesStreamEvent struct {
 type ChatCompletionsRequest struct {
 	Model               string             `json:"model"`
 	Messages            []ChatMessage      `json:"messages"`
+	Instructions        string             `json:"instructions,omitempty"` // OpenAI Responses API compat
 	MaxTokens           *int               `json:"max_tokens,omitempty"`
 	MaxCompletionTokens *int               `json:"max_completion_tokens,omitempty"`
 	Temperature         *float64           `json:"temperature,omitempty"`