Scryfall was returning 429/503 transiently, causing catalog sync to
fail immediately with no recovery. Adds exponential-backoff retry
(3 attempts) for rate-limit and service-unavailable responses in both
the set discovery and card import paths. Also adds proper pagination
support for sets with 175+ cards and URL-encodes set codes.
Co-authored-by: Cursor <cursoragent@cursor.com>