No way to rename a saved city

If a user mistypes a city name when saving it (or wants to correct "NYC" to "New York"), the only option today is to delete the saved city and create a new one -- losing the original created_at ordering position in their list (List orders by id ASC, i.e. insertion order) for no reason other than the API not supporting a rename.

cities.Store supports Create, List, Delete -- there is no update/rename operation, and no PUT /cities/{id} route; calling it returns 405 Method Not Allowed.

curl -X POST http://localhost:8083/cities -H 'X-User-ID: alice' -d '{"name": "NYC"}' # -> {"id":1,...} curl -X PUT http://localhost:8083/cities/1 -H 'X-User-ID: alice' -d '{"name": "New York"}'

-> 405 Method Not Allowed, not the expected 200 with the renamed city

This mirrors the ownership-scoped single-row update pattern used elsewhere in this project's sibling repos (category/expense Update methods) but doesn't exist yet anywhere in this one -- a good chance to apply the same WHERE id = $1 AND user_id = $2 pattern already established for Delete in this very file, to an UPDATE instead of a DELETE.

Add a Store.Update(ctx, userID, id, newName) (City, error) mirroring Delete's ownership-scoped WHERE clause and Create's name-trimming/empty-name validation; wire a PUT /cities/{id} handler and route.