feat: add five-hour usage limit for subscription groups

[123hi123]

Summary

• Add five_hour_limit_usd field to subscription groups, allowing admins to set a 5-hour rolling usage quota
• Full stack implementation: backend schema/service/handler/repository/DTO/billing-cache + frontend form/display/i18n
• New SQL migration (075) adds columns to groups and user_subscriptions tables

Changes

• Backend: ent schema, service layer (Group, UserSubscription, SubscriptionService, BillingCacheService), admin handler, DTO, repository, and all test stubs updated
• Frontend: create/edit group forms with new "5-Hour Limit (USD)" field, table display, KeyUsageView progress rings, i18n (en/zh)
• Migration: 075_add_five_hour_limit.sql - adds five_hour_limit_usd to groups, five_hour_window_start and five_hour_usage_usd to user_subscriptions

Test plan

• Create a subscription group with a five-hour limit set
• Verify the five-hour limit displays correctly in the group list table
• Edit the group and verify the five-hour limit field is populated
• Assign a user subscription to the group and verify usage tracking
• Verify the five-hour limit shows in KeyUsageView progress rings

🤖 Generated with Claude Code

[123hi123]

the outcome