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.
- Update `generate_legal_documents` default `template_dir` from `"templates/legal"` to `"mywebdav/templates/legal"`
- Update `Jinja2Templates` directory from `"templates"` to `"mywebdav/templates"`
- Add new `base.html` template with navigation, footer, and SEO meta tags
- Add new `features.html` template with responsive grid layout and feature cards
- Add new legal document templates: `compliance_statement.html`, `contact_complaint_mechanism.html`, `cookie_policy.html`, `data_portability_deletion_policy.html`
