Feat: Add JWT config, auth middleware, and token parsing using username.

Signed-off-by: Goldbro233 <bowensun_06@outlook.com>

models/ex_rate.go

@@ -0,0 +1,11 @@
+package models
+
+import "time"
+
+type ExchangeRate struct {
+	ID           uint      `gorm:"primary_key" json:"_id"`
+	FromCurrency string    `json:"from_currency" binding:"required"`
+	ToCurrency   string    `json:"to_currency" binding:"required"`
+	Rate         float64   `json:"rate" binding:"required"`
+	Date         time.Time `json:"date"`
+}