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/ratelimit_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/middleware/middlewaretest"
 )
 
 func TestPostRateLimit_AllowsGET(t *testing.T) {
@@ -198,7 +199,7 @@ func rateLimitMiddleware(
 		&slog.HandlerOptions{Level: slog.LevelDebug},
 	))
 
-	return middleware.NewForTest(log, cfg, nil)
+	return middlewaretest.New(t, log, cfg, nil)
 }
 
 // trustedProxies parses CIDR strings for a test Config.