Hash passwords with 1 MiB in tests so make test fits in 4 GiB (closes #261)

On a build machine with 4 GiB, docker build . failed: the auth test
binary was killed for lack of memory. upaasd hashes passwords with
argon2id at 64 MiB per hash; the auth and handlers tests run many such
hashes at once, and the race detector multiplies what each one takes,
so make test peaked at 2.7 GiB.

The memory per hash is now the auth service's ArgonMemory field. New
sets it to the same 64 MiB, which upaasd keeps; the test helpers lower
it to 1 MiB. A new test checks that New sets 64 MiB and hashes and
verifies a password with it. GOMAXPROCS=4 make test now peaks at about
1.4 GiB.

Model: opus-5-5

internal/handlers/handlers_test.go

@@ -109,6 +109,9 @@ func createAppServices(
 	})
 	require.NoError(t, authErr)
 
+	// 1 MiB per password hash instead of 64 MiB; see auth.Service.ArgonMemory.
+	authSvc.ArgonMemory = 1024
+
 	appSvc, appErr := app.New(fx.Lifecycle(nil), app.ServiceParams{
 		Logger:   logInstance,
 		Database: dbInstance,