ci: shard test job + cheap argon2 in test mode

Test job was wall-clocked by `internal/server/http` (~156s on the
self-hosted runner under -race). Two changes here cut that:

1. Matrix-shard the test job by package group: server-http, store,
   and "rest" (everything else, computed via `go list | grep -v`).
   Each shard runs on its own runner so the heavy package isn't
   CPU-starved by siblings.

2. `auth.HashPassword` drops to cheap argon2id params (8 KiB / 1
   iter / 1 lane) when `testing.Testing()` returns true. Production
   params are unchanged. VerifyPassword reads params from the
   encoded hash so cheap-params hashes verify identically — no test
   call sites need to change.

internal/auth/passwords.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+	"testing"
 
 	"golang.org/x/crypto/argon2"
 )
@@ -27,22 +28,38 @@ const (
 	defaultKeyLen     = 32
 )
 
+// Cheap params used only when the binary is a `go test` binary
+// (testing.Testing() == true). Argon2id at production params costs
+// 300–500 ms per hash and dominates wall time on CI runners under
+// `-race`. Tests don't need real KDF strength — VerifyPassword reads
+// params from the encoded hash, so verifying a cheap-params hash
+// works the same way.
+const (
+	testMemoryKiB  = 8
+	testIterations = 1
+	testParallel   = 1
+)
+
 // HashPassword returns an argon2id-encoded string of the form
 //
 //	$argon2id$v=19$m=...,t=...,p=...$<salt>$<hash>
 //
 // safe to store in a TEXT column. The salt is freshly random per call.
 func HashPassword(password string) (string, error) {
+	mem, iter, par := uint32(defaultMemoryKiB), uint32(defaultIterations), uint8(defaultParallel)
+	if testing.Testing() {
+		mem, iter, par = testMemoryKiB, testIterations, testParallel
+	}
 	salt := make([]byte, defaultSaltLen)
 	if _, err := rand.Read(salt); err != nil {
 		return "", fmt.Errorf("auth: read salt: %w", err)
 	}
 	hash := argon2.IDKey([]byte(password), salt,
-		defaultIterations, defaultMemoryKiB, defaultParallel, defaultKeyLen)
+		iter, mem, par, defaultKeyLen)
 
 	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
 		argon2.Version,
-		defaultMemoryKiB, defaultIterations, defaultParallel,
+		mem, iter, par,
 		base64.RawStdEncoding.EncodeToString(salt),
 		base64.RawStdEncoding.EncodeToString(hash),
 	), nil