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/channels/crypto.go

@@ -0,0 +1,63 @@
+package channels
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"io"
+)
+
+// EncryptSecret encrypts plaintext using AES-256-GCM with a random nonce.
+// Returns base64(nonce || ciphertext).
+func EncryptSecret(key [32]byte, plaintext string) (string, error) {
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		return "", fmt.Errorf("aes cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("gcm: %w", err)
+	}
+
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", fmt.Errorf("nonce: %w", err)
+	}
+
+	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(ciphertext), nil
+}
+
+// DecryptSecret decrypts a value produced by EncryptSecret.
+func DecryptSecret(key [32]byte, encoded string) (string, error) {
+	data, err := base64.StdEncoding.DecodeString(encoded)
+	if err != nil {
+		return "", fmt.Errorf("base64 decode: %w", err)
+	}
+
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		return "", fmt.Errorf("aes cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("gcm: %w", err)
+	}
+
+	nonceSize := gcm.NonceSize()
+	if len(data) < nonceSize {
+		return "", fmt.Errorf("ciphertext too short")
+	}
+
+	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return "", fmt.Errorf("decrypt: %w", err)
+	}
+
+	return string(plaintext), nil
+}