[Feature Request] Add /v1/subscription/usage API endpoint for Claude Max plan metrics

Bug Description
As a Claude Max subscriber, I'd like an API endpoint that exposes my current subscription usage metrics — messages sent, tokens consumed, rate limit status, and remaining allocation for the current period. Currently there's no programmatic way to monitor Max plan usage; Claude Code offers /status but that's only available within the terminal, and claude.ai has no usage visibility at all. An endpoint like /v1/subscription/usage (authenticated via OAuth or session token) would allow subscribers to build lightweight tools — desktop widgets, browser extensions, status bar apps — to monitor their usage in real time without having to guess when they're approaching limits. This would be especially valuable for Max users who are paying $100–$200/month and want to pace their usage effectively across the billing period.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.62
• Feedback ID: 827f0273-9d5f-4c24-96a1-d55e79859a5c