Allow digit-prefixed repository owners

[w3lld1]

Summary

Allow UpstreamRepositoryRef.owner values to start with a digit so valid GitLab namespaces such as 2testminder can be registered. The generated protobuf descriptor is updated, and a regression test covers letter-prefixed, digit-prefixed, and invalid punctuation-prefixed owners.

Fixes #6590

Testing

• go test ./pkg/api/protobuf/go/minder/v1 -run TestUpstreamRepositoryRef_Validate -count=1
• buf lint
• make gen
• git diff --check

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 60.923%. remained the same — w3lld1:fix/gitlab-numeric-namespace into mindersec:main

[intelligent-ears]

Thanks for the turnaround on this! The fix looks correct — ^[A-Za-z0-9][-[:word:]]*$ accurately reflects GitLab's actual namespace rules while remaining valid for GitHub (which doesn't allow digit-starting org names anyway).

A few notes:

• Proto change is minimal and correct
• make gen was clearly run — the .pb.go binary descriptor update matches the pattern change
• Test coverage is good with the three cases

One suggestion: it might be worth adding a test case for a digit-hyphen-word style name like 2-mindertest (digit followed by hyphens and words), since that's the real-world pattern that triggered this issue. The current regex should handle it, but an explicit test case would make that clear.
Otherwise this looks good to me. Just needs the CLA to be signed before it can be merged.