Get session secret from env instead of hardcoding

cmd/budgeteer/main.go

@@ -30,8 +30,10 @@ func main() {
 	}
 
 	handler := &server.Handler{
-		Service:             queries,
-		TokenVerifier:       &jwt.TokenVerifier{},
+		Service: queries,
+		TokenVerifier: &jwt.TokenVerifier{
+			Secret: cfg.SessionSecret,
+		},
 		CredentialsVerifier: &bcrypt.Verifier{},
 		StaticFS:            http.FS(static),
 	}