feat: encrypt sensitive values in app.yaml with AES-256-GCM

Passwords and secrets from deploy fields (type: password/secret) are now
encrypted at rest in app.yaml using a per-node 32-byte key. Values stored
as ENC:base64(nonce+ciphertext), decrypted transparently for docker-compose
and web UI. Key included in infra backup bundle for disaster recovery.
Existing plaintext values migrated automatically on startup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/setup/handlers.go

@@ -653,6 +653,16 @@ func (s *Server) restoreFromInfraBackup(ib *report.InfraBackup) {
 			}
 		}
 	}
+
+	// Restore encryption key for app.yaml secrets
+	if ib.EncryptionKeyB64 != "" {
+		if data, err := base64.StdEncoding.DecodeString(ib.EncryptionKeyB64); err == nil {
+			keyFile := filepath.Join(s.dataDir, "encryption.key")
+			if err := atomicWriteFile(keyFile, data, 0600); err != nil {
+				s.logger.Printf("[WARN] Setup: failed to restore encryption key: %v", err)
+			}
+		}
+	}
 }
 
 func (s *Server) writeFreshConfig(configYAML, retrievalPassword string) error {