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/database/webhook_db_manager.go

@@ -15,7 +15,6 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/datadir"
 	"sneak.berlin/go/webhooker/internal/gormlog"
-	"sneak.berlin/go/webhooker/internal/logger"
 )
 
 // WebhookDBManagerParams holds the fx dependencies for
@@ -24,7 +23,7 @@ type WebhookDBManagerParams struct {
 	fx.In
 
 	Config *config.Config
-	Logger *logger.Logger
+	Logger *slog.Logger
 }
 
 // errInvalidCachedDBType indicates a type assertion failure
@@ -70,7 +69,7 @@ func NewWebhookDBManager(
 ) (*WebhookDBManager, error) {
 	m := &WebhookDBManager{
 		dataDir: params.Config.DataDir,
-		log:     params.Logger.Get(),
+		log:     params.Logger,
 	}
 
 	// Create data directory if it doesn't exist. datadir.DirPerm is the