Merge branch 'fix/calculation-accuracy' into main

Combines the decimal migration (int cents → decimal.Decimal via
shopspring/decimal) with main's handler refactor (split space.go into
domain handlers, WithTx/Paginate helpers, recurring deposit removal).

- Repository layer: WithTx pattern + decimal column names/types
- Handler layer: decimal arithmetic (.Sub/.Add) instead of int operators
- Models: deprecated amount_cents fields kept for SELECT * compatibility
- INSERT statements: old columns set to literal 0 for NOT NULL constraints

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

internal/handler/recurring_handler.go

@@ -107,7 +107,7 @@ func (h *RecurringHandler) CreateRecurringExpense(w http.ResponseWriter, r *http
 		ui.RenderError(w, r, "Invalid amount format.", http.StatusUnprocessableEntity)
 		return
 	}
-	amountCents := int(amountDecimal.Mul(decimal.NewFromInt(100)).IntPart())
+	amount := amountDecimal
 
 	startDate, err := time.Parse("2006-01-02", startDateStr)
 	if err != nil {
@@ -176,7 +176,7 @@ func (h *RecurringHandler) CreateRecurringExpense(w http.ResponseWriter, r *http
 		SpaceID:         spaceID,
 		UserID:          user.ID,
 		Description:     description,
-		Amount:          amountCents,
+		Amount:          amount,
 		Type:            expenseType,
 		PaymentMethodID: paymentMethodID,
 		Frequency:       frequency,
@@ -235,7 +235,7 @@ func (h *RecurringHandler) UpdateRecurringExpense(w http.ResponseWriter, r *http
 		ui.RenderError(w, r, "Invalid amount.", http.StatusUnprocessableEntity)
 		return
 	}
-	amountCents := int(amountDecimal.Mul(decimal.NewFromInt(100)).IntPart())
+	amount := amountDecimal
 
 	startDate, err := time.Parse("2006-01-02", startDateStr)
 	if err != nil {
@@ -290,7 +290,7 @@ func (h *RecurringHandler) UpdateRecurringExpense(w http.ResponseWriter, r *http
 	updated, err := h.recurringService.UpdateRecurringExpense(service.UpdateRecurringExpenseDTO{
 		ID:              recurringID,
 		Description:     description,
-		Amount:          amountCents,
+		Amount:          amount,
 		Type:            model.ExpenseType(typeStr),
 		PaymentMethodID: paymentMethodID,
 		Frequency:       model.Frequency(frequencyStr),