feat: add currency (#76)

Resolves #47.

Author: morremeyer

internal/models/budget.go

@@ -6,6 +6,7 @@ package models
 // resources reference it directly or transitively.
 type Budget struct {
 	Model
-	Name string `json:"name,omitempty"`
-	Note string `json:"note,omitempty"`
+	Name     string `json:"name,omitempty"`
+	Note     string `json:"note,omitempty"`
+	Currency string `json:"currency,omitempty"`
 }