Keep test helpers out of the shipped binary (closes #506)

The four testing.go files in config, database, middleware and session
are gone. ClearEnvForTest moves to internal/config/configtest as
ClearEnv; the webhook database manager helpers move to
internal/database/databasetest as NewWebhookDBManager and
NewWebhookDBManagerWithLogger, and the middleware's NewForTest to
internal/middleware/middlewaretest as New, both now built through the
production constructors. Tests that wrapped an open main database use
database.Open. The session helpers move into the session package's
export_test.go; the middleware tests build their session through
session.New and age its timestamps instead of using a fake clock.

The session, middleware and webhook database manager now take the
*slog.Logger they log through, so tests in other packages can give
them their own.

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,