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.
Migrate billing test suite from per-module Tortoise initializer/finalizer fixtures to a shared session-scoped setup in tests/conftest.py, converting all @pytest.fixture async helpers to @pytest_asyncio.fixture and removing the deleted e2e Playwright test directory.
