Add multi-tier subscription system (Starter, Professional, Enterprise) with per-tier storage and bandwidth pricing, replace static free-tier limits with nullable usage-based fields in SubscriptionPlan model, introduce subscribe/unsubscribe API endpoints with plan validation, update invoice generator to calculate costs based on user's active plan tier, and refactor pricing page to load plans dynamically from backend API instead of hardcoded HTML.
Replace every occurrence of "RBox" with "MyWebdav" in source files, configuration,
legal documents, UI components, and test fixtures. This includes renaming the
`RBoxApp` class to `MyWebdavApp` in the JavaScript component, updating the
TOTP issuer from "RBox" to "MyWebdav" in the auth router, changing the
application description in the argument parser, and updating all user-facing
strings in HTML templates, legal policies, and the manifest file. The
`.env.example` default for `TOTP_ISSUER` is also updated accordingly.
