Support `--labs all` to enable every labs feature at once

When running a miren server (especially internal clusters like garden), it's easy to forget to include a newly added labs feature in the --labs flag. This happened with routeoidc on garden — the OIDC provider was properly configured on the socials.miren.garden route, but the middleware was a no-op because routeoidc wasn't in the labs list.

Add support for a special all value (e.g. --labs all or MIREN_LABS=all) that enables every labs feature. This is useful for internal/dogfood clusters where we always want to be running with all features enabled.

Individual features could still be disabled with the existing - prefix syntax, e.g. --labs all,-addons.

Acceptance criteria:

• --labs all enables every defined labs feature
• --labs all,-featurename enables all except the specified feature(s)
• MIREN_LABS=all works equivalently