chore(payment): mark legacy AES ciphertext fallback as deprecated

明文 JSON 已经是新写入的默认格式；保留 AES 密文读取仅为兼容迁移期间的旧
记录，一旦所有部署通过管理后台重存过一次即可删除。标记为 deprecated 并加
TODO，几个版本后统一清理掉：payment.Encrypt / payment.Decrypt、两处
decryptConfig 的 AES 分支、PaymentConfigService.encryptionKey 和
DefaultLoadBalancer.encryptionKey 字段。

backend/internal/payment/crypto.go

@@ -16,6 +16,11 @@ const AES256KeySize = 32
 // Encrypt encrypts plaintext using AES-256-GCM with the given 32-byte key.
 // The output format is "iv:authTag:ciphertext" where each component is base64-encoded,
 // matching the Node.js crypto.ts format for cross-compatibility.
+//
+// Deprecated: payment provider configs are now stored as plaintext JSON.
+// This function is kept only for seeding legacy ciphertext in tests and for
+// the transitional Decrypt fallback. Scheduled for removal after all live
+// deployments complete migration by re-saving their configs.
 func Encrypt(plaintext string, key []byte) (string, error) {
 	if len(key) != AES256KeySize {
 		return "", fmt.Errorf("encryption key must be %d bytes, got %d", AES256KeySize, len(key))
@@ -54,6 +59,11 @@ func Encrypt(plaintext string, key []byte) (string, error) {
 
 // Decrypt decrypts a ciphertext string produced by Encrypt.
 // The input format is "iv:authTag:ciphertext" where each component is base64-encoded.
+//
+// Deprecated: payment provider configs are now stored as plaintext JSON.
+// This function remains only as a read-path fallback for pre-migration
+// ciphertext records. Scheduled for removal once all deployments re-save
+// their provider configs through the admin UI.
 func Decrypt(ciphertext string, key []byte) (string, error) {
 	if len(key) != AES256KeySize {
 		return "", fmt.Errorf("encryption key must be %d bytes, got %d", AES256KeySize, len(key))

backend/internal/payment/load_balancer.go

@@ -283,6 +283,11 @@ func (lb *DefaultLoadBalancer) buildSelection(selected *dbent.PaymentProviderIns
 // Unreadable values (legacy ciphertext without a valid key, or malformed data)
 // are treated as empty so the service keeps running while the admin re-enters
 // the config via the UI.
+//
+// TODO(deprecated-legacy-ciphertext): The AES fallback branch below is a
+// transitional compatibility shim for pre-plaintext records. Remove it (and
+// the encryptionKey field + the Decrypt import) after a few releases once all
+// live deployments have re-saved their provider configs through the UI.
 func (lb *DefaultLoadBalancer) decryptConfig(stored string) (map[string]string, error) {
 	if stored == "" {
 		return nil, nil
@@ -291,7 +296,9 @@ func (lb *DefaultLoadBalancer) decryptConfig(stored string) (map[string]string,
 	if err := json.Unmarshal([]byte(stored), &config); err == nil {
 		return config, nil
 	}
+	// Deprecated: legacy AES-256-GCM ciphertext fallback — scheduled for removal.
 	if len(lb.encryptionKey) == AES256KeySize {
+		//nolint:staticcheck // SA1019: intentional legacy fallback, scheduled for removal
 		if plaintext, err := Decrypt(stored, lb.encryptionKey); err == nil {
 			if err := json.Unmarshal([]byte(plaintext), &config); err == nil {
 				return config, nil

backend/internal/service/payment_config_providers.go

@@ -296,6 +296,10 @@ func (s *PaymentConfigService) mergeConfig(ctx context.Context, id int64, newCon
 // ("iv:authTag:ciphertext"). Values that cannot be parsed as either — including
 // legacy ciphertext with no/invalid TOTP_ENCRYPTION_KEY — are treated as empty,
 // letting the admin re-enter the config via the UI to complete the migration.
+//
+// TODO(deprecated-legacy-ciphertext): The AES fallback branch is a transitional
+// shim for pre-plaintext records. Remove it (and the encryptionKey field) after
+// a few releases once all live deployments have re-saved their provider configs.
 func (s *PaymentConfigService) decryptConfig(stored string) (map[string]string, error) {
 	if stored == "" {
 		return nil, nil
@@ -304,7 +308,9 @@ func (s *PaymentConfigService) decryptConfig(stored string) (map[string]string,
 	if err := json.Unmarshal([]byte(stored), &cfg); err == nil {
 		return cfg, nil
 	}
+	// Deprecated: legacy AES-256-GCM ciphertext fallback — scheduled for removal.
 	if len(s.encryptionKey) == payment.AES256KeySize {
+		//nolint:staticcheck // SA1019: intentional legacy fallback, scheduled for removal
 		if plaintext, err := payment.Decrypt(stored, s.encryptionKey); err == nil {
 			if err := json.Unmarshal([]byte(plaintext), &cfg); err == nil {
 				return cfg, nil