chore(deps): update auth middleware and dependencies

- Add OptionalSessionMiddleware for non-required auth checks
- Add GetUserID helper function
- Update import paths in auth and main
- Update docker-compose with frontend build configuration
- Clean up go.mod and go.sum

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.go

@@ -5,10 +5,10 @@ import (
 	"net/http"
 	"os"
 
-	"github.com/writekitapp/writekit/internal/db"
-	"github.com/writekitapp/writekit/internal/server"
-	"github.com/writekitapp/writekit/internal/storage"
-	"github.com/writekitapp/writekit/internal/tenant"
+	"writekit/internal/db"
+	"writekit/internal/server"
+	"writekit/internal/storage"
+	"writekit/internal/tenant"
 )
 
 func main() {