feat: Add JWT configuration to environment and refactor JWT middleware to use new config structure

backend/internal/models/jwt.go

@@ -0,0 +1,10 @@
+package models
+
+import "time"
+
+type JWTConfig struct {
+	Secret        string        `env:"JWT_SECRET" default:""`
+	TokenDuration time.Duration `env:"JWT_TOKEN_DURATION" default:"24h"`
+	KeyGenerate   bool          `env:"JWT_KEY_GENERATE" default:"false"`
+	KeyDir        string        `env:"JWT_KEY_DIR" default:"./keys"`
+}