feat(wallet): add wallet entities and repository

Entities for the four wallet tables and a WalletRepository that the wallet
processor will build on (PC-103).

Every method goes through the caller's transaction, and writes and locks
refuse to run without one: outside a transaction a lock is released as soon
as it is taken and a balance could move without its ledger row.

- LockWallet creates the wallet on first use, taking the organization from
  the customer, then locks it with SELECT ... FOR UPDATE.
- LockWallets always locks in customer_id order so opposite transfers
  cannot deadlock.
- AddBalance and ConsumeLot are conditional updates that return an error
  when they would overdraw, instead of tripping the CHECK constraint.
- ListActiveLots returns unexpired lots with balance in K9 spending order.

The tests need a real Postgres and run only when TEST_DATABASE_URL points at
a migrated database. Both the lock and the lock ordering were checked by
removing them and watching the tests fail (lost update, deadlock detected).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/constants/wallet.go

@@ -0,0 +1,44 @@
+package constants
+
+// The two balances a customer wallet holds (docs/prd-point-coin.md). EnakPoint pays
+// for orders; EnakCoin is spent on games and can be exchanged into EnakPoint.
+const (
+	WalletCurrencyPoint = "POINT"
+	WalletCurrencyCoin  = "COIN"
+)
+
+func IsValidWalletCurrency(currency string) bool {
+	return currency == WalletCurrencyPoint || currency == WalletCurrencyCoin
+}
+
+// Ledger row types. §8.1 of the PRD lists, per type, which currency it may use, which
+// way it moves the balance, and which reference it must carry.
+const (
+	WalletTxTypeEarn          = "EARN"
+	WalletTxTypeEarnReversal  = "EARN_REVERSAL"
+	WalletTxTypePayment       = "PAYMENT"
+	WalletTxTypePaymentRefund = "PAYMENT_REFUND"
+	WalletTxTypeExchangeOut   = "EXCHANGE_OUT"
+	WalletTxTypeExchangeIn    = "EXCHANGE_IN"
+	WalletTxTypeTransferOut   = "TRANSFER_OUT"
+	WalletTxTypeTransferIn    = "TRANSFER_IN"
+	WalletTxTypeGameSpend     = "GAME_SPEND"
+	WalletTxTypeExpire        = "EXPIRE"
+	WalletTxTypeAdjustment    = "ADJUSTMENT"
+	WalletTxTypeMigration     = "MIGRATION"
+	WalletTxTypeRewardRedeem  = "REWARD_REDEEM"
+)
+
+// What a ledger row's reference_id points at: where the value came from for a
+// credit, or where it went for a debit.
+const (
+	WalletRefTypeOrder            = "ORDER"
+	WalletRefTypePayment          = "PAYMENT"
+	WalletRefTypeWalletTx         = "WALLET_TX"
+	WalletRefTypeGamePlay         = "GAME_PLAY"
+	WalletRefTypeLot              = "LOT"
+	WalletRefTypeUser             = "USER"
+	WalletRefTypeLegacyPoints     = "LEGACY_POINTS"
+	WalletRefTypeLegacyTokens     = "LEGACY_TOKENS"
+	WalletRefTypeRewardRedemption = "REWARD_REDEMPTION"
+)