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/auth/password.go

@@ -0,0 +1,16 @@
+package auth
+
+import "golang.org/x/crypto/bcrypt"
+
+const bcryptCost = 12
+
+// HashPassword returns the bcrypt hash of a plaintext password.
+func HashPassword(plaintext string) (string, error) {
+	b, err := bcrypt.GenerateFromPassword([]byte(plaintext), bcryptCost)
+	return string(b), err
+}
+
+// CheckPassword returns nil if plaintext matches the stored hash.
+func CheckPassword(hash, plaintext string) error {
+	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(plaintext))
+}