feat: v1→v2 wallet key migration on server startup

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

internal/chain/repository.go

@@ -29,3 +29,22 @@ func (r *Repository) FindByPubKeyHex(pubKeyHex string) (*UserWallet, error) {
 	}
 	return &w, nil
 }
+
+// FindAllByKeyVersion returns all wallets with the given key version.
+func (r *Repository) FindAllByKeyVersion(version int) ([]UserWallet, error) {
+	var wallets []UserWallet
+	if err := r.db.Where("key_version = ?", version).Find(&wallets).Error; err != nil {
+		return nil, err
+	}
+	return wallets, nil
+}
+
+// UpdateEncryption updates the encryption fields of a wallet.
+func (r *Repository) UpdateEncryption(id uint, encPrivKey, encNonce, hkdfSalt string, keyVersion int) error {
+	return r.db.Model(&UserWallet{}).Where("id = ?", id).Updates(map[string]any{
+		"encrypted_priv_key": encPrivKey,
+		"enc_nonce":          encNonce,
+		"hkdf_salt":          hkdfSalt,
+		"key_version":        keyVersion,
+	}).Error
+}