Keep test helpers out of the shipped binary (closes #506)

The test helpers lived in ordinary `testing.go` files inside the config, database, middleware and session packages, so they were built into the binary and the shared `test-support` lint rule could not see them. The four files are gone: the session's helpers move into its own `_test.go` file, and the rest into `configtest`, `databasetest` and `middlewaretest`, which the `depguard` deny list now names, so a non-test file importing them fails lint. The test-support packages build through the production constructors.

Judgement call: the session, the middleware and the webhook database manager now take the plain logger they log through, which the application wiring provides.
Judgement call: two idle-expiry tests move the stored timestamps back instead of advancing a fake clock.

Model: opus-5-5

internal/database/databasetest/databasetest.go

@@ -0,0 +1,55 @@
+// Package databasetest builds a WebhookDBManager for tests in other
+// packages.
+package databasetest
+
+import (
+	"log/slog"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// NewWebhookDBManager creates a WebhookDBManager backed by the given
+// data directory, logging at DEBUG to standard error.
+func NewWebhookDBManager(
+	t *testing.T, dataDir string,
+) *database.WebhookDBManager {
+	t.Helper()
+
+	return NewWebhookDBManagerWithLogger(
+		t,
+		dataDir,
+		slog.New(slog.NewTextHandler(
+			os.Stderr,
+			&slog.HandlerOptions{Level: slog.LevelDebug},
+		)),
+	)
+}
+
+// NewWebhookDBManagerWithLogger is NewWebhookDBManager with the
+// logger supplied by the caller. The per-webhook databases this manager
+// opens hand that logger to gormlog, so a test that needs to see the SQL
+// the service emits can capture it.
+//
+// It is built through database.NewWebhookDBManager on a lifecycle that
+// is never started, so nothing closes its databases but the caller.
+func NewWebhookDBManagerWithLogger(
+	t *testing.T, dataDir string, log *slog.Logger,
+) *database.WebhookDBManager {
+	t.Helper()
+
+	mgr, err := database.NewWebhookDBManager(
+		fxtest.NewLifecycle(t),
+		database.WebhookDBManagerParams{
+			Config: &config.Config{DataDir: dataDir},
+			Logger: log,
+		},
+	)
+	require.NoError(t, err)
+
+	return mgr
+}