Add header-only COVID-19 Cases per Country dataset

[cnbrenci]

What changed

Adds dashboard/config/datablock_storage/datasets/covid-19-cases-per-country.json back to datablock storage so projects that still reference COVID-19 Cases per Country can resolve the shared dataset again.

The seed uses the original table name and column shape, with a single placeholder row so the existing CSV importer does not reject it as empty.

Why this changed

The live COVID-19 Cases per Country dataset was removed in 2024 because the upstream source was stale and broken, but some projects still depend on the shared dataset name.

Impact

Projects that still point at this shared dataset can import the table again without restoring the old live country data feed.

Validation

Ran ./tools/hooks/pre-commit.
Verified the JSON parses.
Verified the CSV columns are:
id,Date,Country,Total Confirmed Cases,Total Recovered,Total Deaths.
Verified the seed payload is non-empty for the current importer.

Dataset JSON

{
  "table_name": "COVID-19 Cases per Country",
  "csv": "id,Date,Country,Total Confirmed Cases,Total Recovered,Total Deaths\n1,N/A,N/A,0,0,0\n"
}

[snickell]

we need to get .gitattributes setup so it shows lfs diffs sigh

[snickell]

@cnbrenci I'd encourage not excluding them from LFS (i.e. keep in LFS), altho it stinks on github, the fix I was thinking of was getting everyone setup so at least locally LFS shows diffs on text files, which is a thing, but I have to figure out how to make it a thing that's in the repo not a command everyone has to run (I have it setup locally).

Now that it has one commit that's removed it from LFS on this branch, would you make sure that you [Squash Merge] in the end, or it'll grow the git history.

Sorry mad confusing, LFS is a cool hack, but is kinda a hack.

[cnbrenci]

@cnbrenci I'd encourage not excluding them from LFS (i.e. keep in LFS), altho it stinks on github, the fix I was thinking of was getting everyone setup so at least locally LFS shows diffs on text files, which is a thing, but I have to figure out how to make it a thing that's in the repo not a command everyone has to run (I have it setup locally).

Now that it has one commit that's removed it from LFS on this branch, would you make sure that you [Squash Merge] in the end, or it'll grow the git history.

Sorry mad confusing, LFS is a cool hack, but is kinda a hack.

LOL what happened was I told codex "get .gitattributes setup so it shows lfs diffs in github" and it went off the rails so I had to make it revert the commit that it pushed and now we're here. But yeah definitely going to triple check that I have squash set when I merge.

[cnbrenci]

Added a placeholder row into the dataset to avoid a test breakage with importing a csv (can't import one without any data)