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

frontend/src/api/admin/channels.ts

@@ -49,6 +49,7 @@ export interface Channel {
   status: string
   billing_model_source: string // "requested" | "upstream"
   restrict_models: boolean
+  features_config?: Record<string, unknown>
   group_ids: number[]
   model_pricing: ChannelModelPricing[]
   model_mapping: Record<string, Record<string, string>> // platform → {src→dst}
@@ -66,6 +67,7 @@ export interface CreateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
+  features_config?: Record<string, unknown>
   apply_pricing_to_account_stats?: boolean
   account_stats_pricing_rules?: AccountStatsPricingRule[]
 }
@@ -79,6 +81,7 @@ export interface UpdateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
+  features_config?: Record<string, unknown>
   apply_pricing_to_account_stats?: boolean
   account_stats_pricing_rules?: AccountStatsPricingRule[]
 }