packages.language only written by nuget; SQL filter by language misses everything else

The `language` column on `packages` is only ever assigned by `Ecosystem::Nuget` (from the nuspec `` element, which is a locale like `en-US`, not a programming language). `Package#language` (`app/models/package.rb:182`) falls back to `repo_metadata['language']` when the column is nil, so `pkg.language` returns `"Scala"` in Ruby, but SQL `WHERE language = 'Scala'` matches zero rows for every non-nuget registry.

On maven (620k packages, 462k with `repo_metadata`):

```
Package.where(registry_id: 22, language: 'Scala').count # => 0
Package.where(registry_id: 22).where("repo_metadata->>'language' = 'Scala'").count # => 79314
```

Options: write `repo_metadata['language']` into the column during `update_repo_metadata` and index it, or drop the column and add an expression index on `(registry_id, (repo_metadata->>'language'))`. The nuget locale value should probably move to `metadata` in either case.

Start with app/models/package.rb:182, update_repo_metadata, and the Ecosystem::Nuget assignment described in the issue. Compare the Ruby language fallback with the SQL filtering behavior and determine how the existing column and repo_metadata are intended to interact. Done means language filters return non-NuGet matches while preserving correct NuGet locale handling and suitable query performance.

Written by the indexing model from the issue text.