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/testing.go

@@ -45,3 +45,20 @@ func NewTestWebhookDBManagerWithLogger(
 		log:     log,
 	}
 }
+
+// testArgon2Memory is the Argon2id memory cost, in KiB, that test
+// binaries hash with: 1 MB instead of the shipped 64 MB.
+const testArgon2Memory = 1024
+
+// LowerPasswordHashCostForTest makes HashPassword use a 1 MB Argon2id
+// memory cost instead of the shipped 64 MB, for the rest of the
+// process. Call it from TestMain, before any test runs.
+//
+// Every test that starts a database hashes the bootstrap admin
+// password, and a package runs dozens of those tests in parallel.
+// Under the race detector each 64 MB hash holds about 150 MB resident.
+// VerifyPassword reads the cost from the hash it checks, so
+// verification follows. Production code never calls this.
+func LowerPasswordHashCostForTest() {
+	hashConfig.Memory = testArgon2Memory
+}