Merchant fee estimates in session preflight

backendv1.1.02026-05-19

What changed

When an agent constructs a session delegation, the preflight call now returns an estimated_fees object covering the catalog's known merchant fees, network gas, and Passport's own service margin. Approval pages can show users an itemized "up to ~$X" total instead of just a raw budget cap.

This estimate is informational only — actual fees still come from the merchant at transaction time — but it makes high-budget sessions less surprising and reduces post-approval support questions.

Included changes:

• feat(session): return estimated_fees in preflight response
• feat(catalog): track per-service fee metadata