feat: v1→v2 wallet key migration on server startup

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

internal/chain/service.go

@@ -204,6 +204,44 @@ func (s *Service) decryptPrivKeyV2(cipherHex, nonceHex, saltHex string, userID u
 	return tocrypto.PrivateKey(plaintext), nil
 }
 
+// ---- Wallet Migration ----
+
+// MigrateWalletKeys re-encrypts all v1 wallets using HKDF per-wallet keys.
+// Each wallet is migrated individually; failures are logged and skipped.
+func (s *Service) MigrateWalletKeys() error {
+	wallets, err := s.repo.FindAllByKeyVersion(1)
+	if err != nil {
+		return fmt.Errorf("query v1 wallets: %w", err)
+	}
+	if len(wallets) == 0 {
+		return nil
+	}
+	log.Printf("INFO: migrating %d v1 wallets to v2 (HKDF)", len(wallets))
+	var migrated, failed int
+	for _, uw := range wallets {
+		privKey, err := s.decryptPrivKey(uw.EncryptedPrivKey, uw.EncNonce)
+		if err != nil {
+			log.Printf("ERROR: v1 decrypt failed for walletID=%d userID=%d: %v", uw.ID, uw.UserID, err)
+			failed++
+			continue
+		}
+		cipherHex, nonceHex, saltHex, err := s.encryptPrivKeyV2(privKey, uw.UserID)
+		if err != nil {
+			log.Printf("ERROR: v2 encrypt failed for walletID=%d userID=%d: %v", uw.ID, uw.UserID, err)
+			failed++
+			continue
+		}
+		if err := s.repo.UpdateEncryption(uw.ID, cipherHex, nonceHex, saltHex, 2); err != nil {
+			log.Printf("ERROR: DB update failed for walletID=%d userID=%d: %v", uw.ID, uw.UserID, err)
+			failed++
+			continue
+		}
+		migrated++
+	}
+	log.Printf("INFO: wallet migration complete: %d migrated, %d failed", migrated, failed)
+	return nil
+}
+
 // ---- Wallet Management ----
 
 // CreateWallet generates a new keypair, encrypts it, and stores in DB.