Implement a full-featured admin backend at `/manage/` with login authentication using
signed cookies and constant-time credential verification. The panel includes a dashboard
showing total/active users, storage usage, monthly revenue, and pending invoices, plus
user management, payment overview, and pricing configuration pages. New environment
variables `ADMIN_USERNAME`, `ADMIN_PASSWORD`, `ADMIN_SESSION_SECRET`, and
`ADMIN_SESSION_EXPIRE_HOURS` control access. The admin router is registered in main.py
and all new templates, auth module, and router files are added.
- Replace "commercial" with "self-hosted" in project description
- Remove Stripe billing integration from admin features list
- Remove Stripe payment credentials from environment variables section
Replace every occurrence of "RBox" with "MyWebdav" in README.md, invoice templates, welcome emails, share notifications, FastAPI metadata, default database filename, domain name, S3 bucket name, TOTP issuer, and WebDAV realm header to reflect the commercial rebranding of the application.
