Refactored to maintain a separate cloud version

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.

pkg/cpserver/options.go

@@ -0,0 +1,27 @@
+package cpserver
+
+// options holds the configuration for Run.
+type options struct {
+	version    string
+	commit     string
+	extensions []Extension
+}
+
+// Option configures the control plane server.
+type Option func(*options)
+
+// WithVersion sets the version and commit strings for logging.
+func WithVersion(version, commit string) Option {
+	return func(o *options) {
+		o.version = version
+		o.commit = commit
+	}
+}
+
+// WithExtensions registers one or more extensions that add routes and
+// background workers to the control plane.
+func WithExtensions(exts ...Extension) Option {
+	return func(o *options) {
+		o.extensions = append(o.extensions, exts...)
+	}
+}