Get Subscription Status

Fast status check - queries DynamoDB only (no Stripe API calls). Use this for polling and subscription guards.

HasActiveSubscription includes BypassSubscriptionCheck so the frontend can rely on this single field for access decisions.

NOTE: This is def (not async def) because it only does synchronous PynamoDB I/O. FastAPI automatically runs def handlers in a thread pool, so they don't block the event loop or other concurrent requests.