Use date_trunc instead of splitting date into year and month

postgres/models.go

@@ -25,8 +25,7 @@ type Assignment struct {
 }
 
 type AssignmentsByMonth struct {
-	Year       float64
-	Month      float64
+	Date       time.Time
 	CategoryID uuid.UUID
 	BudgetID   uuid.UUID
 	Amount     int64
@@ -68,8 +67,7 @@ type Transaction struct {
 }
 
 type TransactionsByMonth struct {
-	Year       float64
-	Month      float64
+	Date       time.Time
 	CategoryID uuid.NullUUID
 	BudgetID   uuid.UUID
 	Amount     int64