feat(commands): implement :CP open [problem|contest|standings]

Problem: There was no way to open a problem or contest page in the
browser from within the plugin; users had to manually navigate to
the platform URL.

Solution: Add `contest_url` and `standings_url` to `MetadataResult`
and persist them via `cache.set_contest_data`. Add `cache.get_open_urls`
to assemble all three URLs from cache. Wire up `:CP open` in
`commands/init.lua` to call `vim.ui.open` on the resolved URL,
warning when none is available (e.g. CSES standings).

scrapers/models.py

@@ -42,6 +42,8 @@ class MetadataResult(ScrapingResult):
     contest_id: str = ""
     problems: list[ProblemSummary] = Field(default_factory=list)
     url: str
+    contest_url: str = ""
+    standings_url: str = ""
 
     model_config = ConfigDict(extra="forbid")