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/delivery/target_database_rotation_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/database/databasetest"
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
@@ -335,7 +336,7 @@ func TestArchivePathAt(t *testing.T) {
 	t.Parallel()
 
 	dataDir := t.TempDir()
-	dbMgr := database.NewTestWebhookDBManager(dataDir)
+	dbMgr := databasetest.NewWebhookDBManager(t, dataDir)
 	webhook := &database.Webhook{
 		BaseModel: database.BaseModel{ID: "wh-id"}, Name: "Orders",
 	}