feat(race): add supports_countdown to ContestListResult

Problem: `:CP race` on platforms without future contests (CSES, USACO)
wastes time fetching the full contest list only to discover there is
no `start_time`. The error message is also uninformative.

Solution: Add `supports_countdown` bool to `ContestListResult` (default
`True`). CSES and USACO set it to `False`. Cache the flag per-platform
so subsequent calls skip the fetch entirely. `race.lua` checks the
cached value first, then the scraper result, and shows
`"<Platform> does not support :CP race"` instead of a generic error.

scrapers/models.py

@@ -50,6 +50,7 @@ class MetadataResult(ScrapingResult):
 
 class ContestListResult(ScrapingResult):
     contests: list[ContestSummary] = Field(default_factory=list)
+    supports_countdown: bool = True
 
     model_config = ConfigDict(extra="forbid")