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/archive_sweeper_test.go

@@ -21,6 +21,7 @@ import (
 	"gorm.io/gorm/clause"
 	_ "modernc.org/sqlite" // Pure Go SQLite driver.
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/database/databasetest"
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 )
@@ -59,29 +60,14 @@ func setupArchiveTest(t *testing.T) *archiveEnv {
 	dataDir := t.TempDir()
 	log := archiveTestLogger()
 
-	sqlDB, err := sql.Open(
-		"sqlite",
-		fmt.Sprintf(
-			"file:%s?mode=rwc",
-			filepath.Join(dataDir, "main.db"),
-		),
-	)
+	mainDB, err := database.Open(dataDir, slog.New(slog.DiscardHandler))
 	require.NoError(t, err)
 
-	t.Cleanup(func() { _ = sqlDB.Close() })
-
-	gdb, err := gorm.Open(
-		sqlite.Dialector{Conn: sqlDB},
-		&gorm.Config{Logger: gormlog.New(slog.New(slog.DiscardHandler))},
-	)
-	require.NoError(t, err)
-
-	mainDB := database.NewTestDatabase(gdb)
-	require.NoError(t, mainDB.Migrate())
+	t.Cleanup(func() { _ = mainDB.Close() })
 
 	eng := delivery.NewTestEngineWithDB(
 		mainDB,
-		database.NewTestWebhookDBManager(dataDir),
+		databasetest.NewWebhookDBManager(t, dataDir),
 		log,
 		&http.Client{Timeout: 5 * time.Second},
 		1,