feature: render RFT promo discount in prime train models

[JannikSt]

Summary

• prime train models now shows discounted prices alongside originals (struck through), with a FREE chip on 100% off categories
• Reads new effective_*_price_per_mtok and promo_label fields from /rft/models
• Falls back to today's rendering when the backend doesn't return promo fields

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Note

Low Risk
UI/formatting-only CLI changes with added test coverage and safe fallback when promo fields are absent; minimal impact beyond model listing output.

Overview
Updates the hosted training models API schema to accept effective_*_price_per_mtok and promo_label from /rft/models, and reflects these fields in CLI JSON output/help.

Improves prime train/rl models table rendering to show discounted pricing as original → effective (with strikethrough + FREE for 100% off) and prints any promo labels once below the table. Adds format_promo_price plus targeted tests covering promo rendering, edge cases, label de-duplication, and JSON field presence.

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