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/handlers/target_create_query_test.go

@@ -15,7 +15,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
-	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/middleware/middlewaretest"
 )
 
 // targetSecretSegments are the path segments of an incoming-webhook
@@ -65,7 +65,8 @@ func postTargetCreate(
 	t.Helper()
 
 	logBuf := new(bytes.Buffer)
-	mw := middleware.NewForTest(
+	mw := middlewaretest.New(
+		t,
 		slog.New(slog.NewJSONHandler(
 			logBuf, &slog.HandlerOptions{Level: slog.LevelInfo},
 		)),