Add the main public method calculate_renewal_price to PricingEngine,
routing to _calculate_tariff_mode or _calculate_classic_mode based on
whether the subscription has a linked tariff. Both modes apply stacked
discounts (promo-group then promo-offer). Classic mode tries
CLASSIC_PERIOD_PRICES first, falling back to PERIOD_PRICES. Adds 8
new tests covering both modes, discounts, extra devices, and fallback.
_calculate_servers_price ALWAYS uses real server.price_kopeks even when
is_available=False or is_full=True, fixing the silent zero-price bug.
_calculate_traffic_price separates base from purchased GB to prevent
purchased top-ups from inflating the tier lookup.
Fringg