Add tests for web dependency pages and restore COVER_MIN to 75

[andrew]

Adds internal/http/web/deps_pages_test.go covering /dependencies, /dependencies/blame, /dependencies/stats, and /dependencies/{purl}. The fixture seeds a two-commit repo with a go.mod, reindexes through a pkgs.Store injected via WithPkgsStore, and asserts status, content type, and body content. Also covers the indexing banner and purlPackageName.

Overall ./internal/... coverage 73.8% → 77.5%, so COVER_MIN goes back to 75.

The tests surfaced a bug in purlPackageName: it cut at the first @, so npm scoped packages like pkg:npm/%40babel/core@7.24.0 returned an empty name. Replaced the hand-rolled parser with purl.Parse(s).FullName() from git-pkgs/purl, which also gives the right separator per ecosystem (e.g. org.junit:junit for maven). indexByteFrom is removed.