Add min_from_amount and candidates to the seam rate quote

[LandynDev]

Seam change for the Allways Access public API v1 (allways-access docs/specs/public-api.md §9).

GET /rate now carries, on 200 and 404:

• min_from_amount: the pair's hub minimum in source units (converted at the best level rate when the source is the spoke; 0 with no depth).
• candidates: up to 5 bound miners in select_best_miner order, each with the executable to_amount for the asked size and its max_from_amount. candidates[0] is the quote.

Bindings resolve before the cap, so an unbound best miner no longer turns a quotable pair into a 404. Older consumers ignore both fields.