No way to remove all of your saved cities at once

A user who wants to start their saved-city list over (or is resetting a demo/test account) has to DELETE /cities/{id} one at a time for every city they've saved -- tedious, and it's an extra round trip per city for something that's conceptually one action ("clear my list").

cities.Store supports deleting one city by id (Delete(ctx, userID, id)). There is no bulk-delete operation and no DELETE /cities (collection-level) route -- calling it returns 405 Method Not Allowed.

curl -X POST http://localhost:8083/cities -H 'X-User-ID: alice' -d '{"name": "London"}' curl -X DELETE http://localhost:8083/cities -H 'X-User-ID: alice'

-> 405 Method Not Allowed (no collection-level DELETE /cities route is registered)

This is a genuinely different SQL shape from the existing single-row Delete -- a DELETE ... WHERE user_id = $1 with no id condition at all, still scoped correctly by user, still using RowsAffected() to report how many rows were actually removed (same pattern as the single-row delete, just without the id clause).

Add a Store method (e.g. DeleteAll(ctx, userID) (int64, error)) mirroring Delete's shape minus the id condition; wire a DELETE /cities route and handler alongside the existing DELETE /cities/{id} one in server.go/handlers.go.