GitLab

- Fix websearch provider failover: proxy error from provider-specific proxy
  now continues to next provider instead of aborting the entire loop
- Fix SMTP failure locking users out: send email first, then write cache
  and increment rate counter
- Fix notify email cache key case sensitivity: normalize to lowercase
- Add OriginalPrice validation to validatePlanPatch and validatePlanRequired
- Add empty scope validation for channel pricing rules (group_ids/account_ids)
- Add platform color to account search dropdown in channel pricing rules

fix: batch 1 audit fixes — quota SQL fixed mode, public recharge URL, WebSearch bool fallback, UpdatePlan validation

H1: incrementUsageBillingAccountQuota now uses shared dailyExpiredExpr/weeklyExpiredExpr
    constants (supporting fixed reset mode) instead of hardcoded '24 hours'/'168 hours'
H4: public settings endpoint now maps balance_low_notify_recharge_url
H6: GetWebSearchEmulationMode tolerates legacy bool values (true→enabled)
H7: UpdatePlan validates non-nil patch fields (rejects negative price, empty name, etc.)
H8: UsageTable accountBilled() helper with total_cost ?? 0 null guard
H9: AdminUsageLog TS type adds channel_id + billing_tier
M2: account.go "fixed" literals replaced with thresholdTypeFixed constant
M13: SystemSettings TS type adds web_search_emulation_enabled
UI: QuotaLimitCard title labels now use flex-1 to align with flex-1 input boxes

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.