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

Every test that starts a database hashed the bootstrap admin password
with Argon2id at 64 MB, which under -race holds about 150 MB per hash,
across dozens of parallel tests. The test packages that hash now lower
the memory cost to 1 MB from TestMain. One test still hashes and
verifies at the shipped parameters, which are unchanged.

script/test also runs at most four packages and eight parallel tests
at once: unbounded, a many-core host linked and ran every test binary
together.

Model: opus-5-5

internal/database/password_test.go

@@ -192,6 +192,36 @@ func TestHashPasswordUniqueness(t *testing.T) {
 	}
 }
 
+// TestHashPassword_ShippedParameters hashes and verifies at the
+// shipped Argon2id parameters. Every other test hashes at the lowered
+// memory cost TestMain sets, so this is the one that keeps production
+// hashing covered. One hash and one verification: each costs 64 MB.
+func TestHashPassword_ShippedParameters(t *testing.T) {
+	t.Parallel()
+
+	password := "correct horse battery staple"
+
+	hash, err := database.HashPasswordWithDefaultsForTest(password)
+	if err != nil {
+		t.Fatalf("hashing with the shipped parameters: %v", err)
+	}
+
+	const shipped = "$argon2id$v=19$m=65536,t=1,p=4$"
+
+	if !strings.HasPrefix(hash, shipped) {
+		t.Errorf("hash = %q, want prefix %q", hash, shipped)
+	}
+
+	valid, err := database.VerifyPassword(password, hash)
+	if err != nil {
+		t.Fatalf("VerifyPassword() error = %v", err)
+	}
+
+	if !valid {
+		t.Error("VerifyPassword() returned false for correct password")
+	}
+}
+
 // TestVerifyDummyPassword_DoesRealWork covers the anti-enumeration
 // path. Login charges an unknown username a verification against a
 // dummy hash so that a nonexistent account is not answered in