Moves 12 packages from internal/ to pkg/ (config, id, validate, events, db,
auth, lifecycle, scheduler, channels, audit, service) so they can be imported
by the enterprise repo as a Go module dependency.
Introduces pkg/cpextension (shared Extension interface + ServerContext) and
pkg/cpserver (Run() entrypoint with functional options) so the enterprise
main.go can call cpserver.Run(cpserver.WithExtensions(...)) without duplicating
the 20-step server bootstrap. Adds db/migrations/embed.go for go:embed access
to OSS SQL migrations from the enterprise module.
cmd/control-plane/main.go is reduced to a 10-line wrapper around cpserver.Run.
Admin users page at /admin/users with paginated user list showing name,
email, team counts, role, join date, and active status toggle. Inactive
users are blocked from all authenticated endpoints immediately via DB
check in JWT middleware. OAuth login errors now show human-readable
messages on the login page.
