fix: coerce null project fields to empty string to avoid NOT NULL violation

src/fabledassistant/routes/projects.py

@@ -88,7 +88,7 @@ async def update_project_route(project_id: int):
     uid = get_current_user_id()
     data = await request.get_json()
     allowed = {"title", "description", "goal", "status", "color"}
-    fields = {k: v for k, v in data.items() if k in allowed}
+    fields = {k: (v if v is not None else "") for k, v in data.items() if k in allowed}
     if "status" in fields and fields["status"] not in ("active", "paused", "completed", "archived"):
         return jsonify({"error": "status must be 'active', 'completed', or 'archived'"}), 400
     project = await update_project(uid, project_id, **fields)