Washington Post downloader: limited date range and Sunday-only constraint

[Ugleh]

The Washington Post (wp) downloader appears to have two issues:

1. Sunday-only puzzles: All requests hit the endpoint /crossword/levels/sunday/YYYY/MM/DD, meaning only Sunday puzzles can be fetched. Passing a non-Sunday date results in No parseable JSON since the API has no Sunday puzzle for that date.
2. Limited historical availability: Even for valid Sunday dates, the API only returns data going back to approximately May 2025. Any Sunday before that fails with the same No parseable JSON error.

Steps to reproduce:

# Non-Sunday date - fails
xword-dl wp -d 2025-12-01
# No parseable JSON at https://games-service-prod.site.aws.wapo.pub/crossword/levels/sunday/2025/12/01

# Valid Sunday but older than ~May 2025 - fails
xword-dl wp -d 2025-04-27
# No parseable JSON at https://games-service-prod.site.aws.wapo.pub/crossword/levels/sunday/2025/04/27

# Valid recent Sunday -  works
xword-dl wp -d 2025-12-28
# Downloads successfully

# Latest - works
xword-dl wp -l
# Downloads successfully

xword-dl version: 2025.10.14
OS: Windows
Python: 3.10