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

script/test

@@ -22,13 +22,18 @@
 # The one figure above 90s is GOMAXPROCS 1, a synthetic core floor rather than
 # a condition CI runs under. If a CPU-limited runner ever puts a real run near
 # 67s, that is the datum to revisit the org figure with.
+#
+# -p 4 -parallel 8 keep the run under 2 GB of memory: at most four test
+# binaries build or run at once, each with at most eight parallel tests. Under
+# -race every test binary and every link costs a few hundred MB, so the
+# defaults (one per core) add up to several GB on a many-core host.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    go test -v -race -timeout 90s ./...
+    go test -v -race -p 4 -parallel 8 -timeout 90s ./...
 }
 
 main "$@"