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/password.go

@@ -9,6 +9,7 @@ import (
 	"math/big"
 	"strings"
 	"sync"
+	"testing"
 
 	"golang.org/x/crypto/argon2"
 )
@@ -63,10 +64,30 @@ func DefaultPasswordConfig() *PasswordConfig {
 	}
 }
 
-// HashPassword generates an Argon2id hash of the password
+// testArgon2Memory is the Argon2id memory cost, in KiB, that a test
+// binary hashes with: 1 MB instead of the shipped 64 MB. Every test
+// that starts a database hashes the bootstrap admin password, dozens
+// of them run in parallel, and under the race detector each 64 MB hash
+// holds about 150 MB. VerifyPassword reads the cost from the hash it
+// checks, so verification follows.
+const testArgon2Memory = 1024
+
+// hashAtShippedCostInTest makes a test binary hash at the shipped
+// memory cost. Only TestHashPassword_ShippedParameters sets it.
+//
+//nolint:gochecknoglobals // set by one test, see above
+var hashAtShippedCostInTest bool
+
+// HashPassword generates an Argon2id hash of the password. A binary
+// built by go test hashes at testArgon2Memory; one built by go build
+// always hashes at the defaults.
 func HashPassword(password string) (string, error) {
 	config := DefaultPasswordConfig()
 
+	if testing.Testing() && !hashAtShippedCostInTest {
+		config.Memory = testArgon2Memory
+	}
+
 	// Generate a salt
 	salt := make([]byte, config.SaltLen)