Price a release before submitting it

Body: { tracks: [...], token_offset?: boolean } — the same tracks you are about to submit. Prices them with the same code that debits on submit, against the live balance, without creating or charging anything. Returns the per-track breakdown, cost.totalCredits, balance.covered, balance.shortfallCredits, and the cheapest bundle covering the shortfall. Gate your submit button on balance.covered rather than recomputing the cost client-side: an estimate that drifts one credit high silently blocks a user whose balance is sufficient.