fix: require admin permission for repository deletion

[immanuwell]

Fixes #10

A write token with admin = false could delete the whole repo.
One DELETE and repo gone

Repro before this fix:

1. Configure a token with write = true, admin = false
2. Create a repo with that token
3. Call DELETE /owner/repo/api
4. It returns 204; the next GET returns 404

Deletion now uses the existing admin check.
Creation still needs write permission.
Added coverage for the root, API, and browser routes, plus updated the docs

Tests:
just web-build
just test
just e2e
just warnings