feat(commands): implement :CP open [problem|contest|standings] (#319)

## Problem

There was no way to open a problem, contest, or standings page in the
browser from within the plugin.

## Solution

Add `contest_url` and `standings_url` to `MetadataResult` and persist
them in the cache. Add `cache.get_open_urls` to resolve all three URLs.
Wire up `:CP open [problem|contest|standings]` in `commands/init.lua`
to call `vim.ui.open`, warning when a URL is unavailable (e.g. CSES
has no standings). Closes #315.

scrapers/atcoder.py

@@ -606,6 +606,8 @@ class AtcoderScraper(BaseScraper):
                 contest_id=contest_id,
                 problems=problems,
                 url=f"https://atcoder.jp/contests/{contest_id}/tasks/{contest_id}_%s",
+                contest_url=f"https://atcoder.jp/contests/{contest_id}",
+                standings_url=f"https://atcoder.jp/contests/{contest_id}/standings",
             )
         except Exception as e:
             return self._metadata_error(str(e))

scrapers/codeforces.py

@@ -223,6 +223,8 @@ class CodeforcesScraper(BaseScraper):
                 contest_id=contest_id,
                 problems=problems,
                 url=f"https://codeforces.com/contest/{contest_id}/problem/%s",
+                contest_url=f"https://codeforces.com/contest/{contest_id}",
+                standings_url=f"https://codeforces.com/contest/{contest_id}/standings",
             )
         except Exception as e:
             return self._metadata_error(str(e))

scrapers/cses.py

@@ -218,6 +218,8 @@ class CSESScraper(BaseScraper):
             contest_id=contest_id,
             problems=problems,
             url="https://cses.fi/problemset/task/%s",
+            contest_url="https://cses.fi/problemset",
+            standings_url="",
         )
 
     async def scrape_contest_list(self) -> ContestListResult:

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")