Implement API features and persistence

[cjo93]

Summary

• wire the worker to log requests, enforce rate limits, classify text, handle Stripe webhooks, and stream R2 mandala assets
• add hashing utilities plus classifier logic, D1 persistence helpers, and comprehensive Jest coverage with test doubles
• document the updated API surface including classification, mandala storage, and billing webhook endpoints

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_b_68f2fce28ca48331a7f699e25a51166f