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/session/export_test.go

@@ -0,0 +1,45 @@
+package session
+
+import (
+	"log/slog"
+	"time"
+
+	"github.com/gorilla/sessions"
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// NewStore exposes the production cookie-store constructor so tests
+// exercise the store the application actually runs with, rather than a
+// lookalike assembled in the test.
+func NewStore(key []byte) *sessions.CookieStore {
+	return newStore(key)
+}
+
+// NewForTest creates a Session with a pre-configured cookie store for use
+// in tests. This bypasses the fx lifecycle and database dependency. The key
+// parameter is the raw 32-byte authentication key used for session encryption
+// and CSRF cookie signing.
+//
+// The idle timeout is taken from cfg.SessionIdleTimeout, exactly as in
+// production. The now parameter supplies the clock used for expiry
+// checks so tests can advance time without sleeping; pass nil for the
+// real clock.
+func NewForTest(
+	store *sessions.CookieStore,
+	cfg *config.Config,
+	log *slog.Logger,
+	key []byte,
+	now func() time.Time,
+) *Session {
+	if now == nil {
+		now = time.Now
+	}
+
+	return &Session{
+		store:       store,
+		key:         key,
+		log:         log,
+		idleTimeout: cfg.SessionIdleTimeout,
+		now:         now,
+	}
+}