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/handlers_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"html/template"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"sync"
@@ -249,6 +250,7 @@ func newTestAppWithConfig(
 		fx.Provide(
 			globals.New,
 			logger.New,
+			func(l *logger.Logger) *slog.Logger { return l.Get() },
 			func() *config.Config { return cfg },
 			database.New,
 			database.NewWebhookDBManager,