Port beta's multiplicative spot-price adjustment (spot_multiplier) to main

[johanzander]

Context

Split out from #218 (item 1). main's entsoe provider (#208) only supports the additive markup model as Nordpool ((spot + markup) * VAT + fees). Beta built out spot_multiplier / export_spot_multiplier fields to support contracts that scale the raw spot price instead (e.g. Belgian Luminus Dynamic: spot * 1.0175 + fees) * VAT, export spot * 1.018 + compensation). Without this, Luminus-style contract users get systematically wrong prices.

This has already been ported onto beta's reset history (bess-manager-beta v9.10.0b1) — that diff is a ready-made reference for porting to main.

Touches

• core/bess/settings.py (PriceSettings.spot_multiplier/export_spot_multiplier)
• core/bess/price_manager.py (_calculate_buy_price/_calculate_sell_price)
• core/bess/battery_system_manager.py (wiring)
• backend/api_dataclasses.py
• backend/api.py (_PROVIDER_PRICING_DEFAULTS / _pricing_defaults_for_discovery)
• frontend/src/components/settings/PricingFormSection.tsx (provider-aware form + help text)
• frontend/src/pages/SetupWizardPage.tsx
• frontend/src/pages/SettingsPage.tsx

Note

#218 items 2 and 3 (setup wizard silently dropping spotMultiplier/exportSpotMultiplier, and Solcast entity-registry detection) are being fixed independently in a separate PR. Once this feature lands, double check the _PRICE_MAP allow-list in backend/api.py's setup_complete() includes the new fields (the contract test pattern from beta's TestPriceModelAttrsConsistency in backend/tests/test_settings_contracts.py is a good reference to prevent this class of bug recurring).

Related: #126, #218

[bess-manager-claude-bot]

Feature request received. This is a port of beta's multiplicative spot-price model to support contracts like Luminus Dynamic that scale the raw spot price rather than adding a fixed markup. The scope is clear and references ready-made diffs on beta. Queued for implementation review.