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/middleware/middlewaretest/middlewaretest.go

@@ -0,0 +1,42 @@
+// Package middlewaretest builds a Middleware for tests in other
+// packages.
+package middlewaretest
+
+import (
+	"log/slog"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/session"
+)
+
+// New builds a Middleware through middleware.New, on a
+// lifecycle that is never started.
+//
+// Its metrics recorder writes to a fresh registry of its own, so
+// Metrics() works on it and two of them never collide.
+func New(
+	t *testing.T,
+	log *slog.Logger,
+	cfg *config.Config,
+	sess *session.Session,
+) *middleware.Middleware {
+	t.Helper()
+
+	m, err := middleware.New(
+		fxtest.NewLifecycle(t),
+		middleware.MiddlewareParams{
+			Logger:   log,
+			Config:   cfg,
+			Session:  sess,
+			Registry: prometheus.NewRegistry(),
+		},
+	)
+	require.NoError(t, err)
+
+	return m
+}