feat(wallet): add wallet engine as the only way to change a balance

WalletProcessor writes the balance, the ledger row and the lots or
allocations together, which keeps SUM(ledger) = balance = SUM(lot
remaining) (docs/prd-point-coin.md §7.5, PC-104).

- Credit writes the ledger row and creates lots, each with its own expiry
  and origin lot.
- Debit draws from the preferred lots first (a reversal's own lots, or the
  lot being expired), then from unexpired lots in K9 order, and returns the
  allocations with their expiry so CarryOver can give the receiving side of
  a transfer or exchange the same expiry.
- DebitUpTo takes what the wallet has and reports the shortfall (F10, Q3).
- An idempotency key returns the first result; reusing it for a different
  operation is an error.
- §8.1 is checked in code from one rule table, ahead of the database
  constraints, so callers get a readable error.

Each method locks the wallet itself, after validating the input and before
checking the idempotency key, so correctness does not depend on the caller.
Operations on two wallets still call LockWallets first to keep lock order.

Unit tests run on an in-memory repository and check the §7.5 invariants
after every scenario; one more test runs the engine against Postgres when
TEST_DATABASE_URL is set.

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

internal/app/app.go

@@ -247,6 +247,7 @@ type repositories struct {
 	productOutletPriceRepo         *repository.ProductOutletPriceRepositoryImpl
 	expenseRepo                    *repository.ExpenseRepositoryImpl
 	cashAdvanceRepo                *repository.CashAdvanceRepositoryImpl
+	walletRepo                     repository.WalletRepository
 }
 
 func (a *App) initRepositories() *repositories {
@@ -302,6 +303,7 @@ func (a *App) initRepositories() *repositories {
 		productOutletPriceRepo:         repository.NewProductOutletPriceRepositoryImpl(a.db),
 		expenseRepo:                    repository.NewExpenseRepositoryImpl(a.db),
 		cashAdvanceRepo:                repository.NewCashAdvanceRepositoryImpl(a.db),
+		walletRepo:                     repository.NewWalletRepository(a.db),
 	}
 }
 
@@ -350,6 +352,7 @@ type processors struct {
 	productOutletPriceProcessor         processor.ProductOutletPriceProcessor
 	expenseProcessor                    *processor.ExpenseProcessorImpl
 	cashAdvanceProcessor                *processor.CashAdvanceProcessorImpl
+	walletProcessor                     *processor.WalletProcessor
 }
 
 func (a *App) initProcessors(cfg *config.Config, repos *repositories) *processors {
@@ -403,6 +406,7 @@ func (a *App) initProcessors(cfg *config.Config, repos *repositories) *processor
 		productOutletPriceProcessor:         processor.NewProductOutletPriceProcessorImpl(repos.productOutletPriceRepo, repos.productRepo, repos.outletRepo),
 		expenseProcessor:                    processor.NewExpenseProcessorImpl(repos.expenseRepo, repos.purchaseCategoryRepo, repos.cashAdvanceRepo),
 		cashAdvanceProcessor:                processor.NewCashAdvanceProcessorImpl(repos.cashAdvanceRepo, repos.categoryRepo),
+		walletProcessor:                     processor.NewWalletProcessor(repos.walletRepo),
 	}
 }