Keep make test under 2 GB of memory (closes #344)

Every test that starts a database seeds the admin account, hashing its password with Argon2id at 64 MB, about 150 MB under -race, and internal/handlers and internal/database ran dozens of those at once. That was the memory, and most of internal/handlers' run time; the product code does not leak.

HashPassword now hashes at a 1 MB cost only when testing.Testing() reports a test binary, so every test package gets it with nothing to add and a binary built by go build always hashes at the shipped parameters. TestHashPassword_ShippedParameters still hashes and verifies through HashPassword at the shipped cost. script/test adds -p 4 -parallel 8.

Model: opus-5-5

internal/database/export_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"log/slog"
 	"os"
+	"testing"
 	"time"
 
 	"go.uber.org/fx"
@@ -79,3 +80,14 @@ func (d *Database) ExportSetBannerOut(w io.Writer) {
 func DummyPasswordHashForTest() string {
 	return dummyPasswordHash()
 }
+
+// HashAtShippedCostForTest makes HashPassword hash at the shipped
+// memory cost until t ends. t must not run in parallel with other
+// tests, which would hash at that cost alongside it.
+func HashAtShippedCostForTest(t *testing.T) {
+	t.Helper()
+
+	hashAtShippedCostInTest = true
+
+	t.Cleanup(func() { hashAtShippedCostInTest = false })
+}