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; the
webhook database manager helpers move to internal/database/databasetest
and the middleware's NewForTest to internal/middleware/middlewaretest,
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/server/routes_test.go

@@ -3,6 +3,7 @@ package server_test
 import (
 	"context"
 	"html"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -130,6 +131,7 @@ func newTestEnvWithConfig(
 		fx.Provide(
 			globals.New,
 			logger.New,
+			func(l *logger.Logger) *slog.Logger { return l.Get() },
 			func() *config.Config { return cfg },
 			database.New,
 			database.NewWebhookDBManager,