No way to fetch a single saved city by id

A client rendering a single saved city (a "city details" view, or confirming a delete before it happens) has no way to fetch one city's record -- it has to fetch and filter the caller's entire saved-city list client-side just to check whether one id still exists and belongs to them, wasted work that won't scale if a user saves many cities.

cities.Store has Create, List, Delete -- no Get. No route for GET /cities/{id} exists; today that URL returns 405 Method Not Allowed (DELETE /cities/{id} is already registered, so the mux recognizes the path but not GET).

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

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

This is the same ownership-scoping discipline the rest of the project applies to weather-fetching concurrency and cache correctness, applied here to a plain CRUD read -- the WHERE clause (id = $1 AND user_id = $2) is the enforcement point, mirroring cities.Store.Delete a few lines above where this method belongs.

Add a Store method alongside Delete with the same two-argument shape (userID, id) and an analogous WHERE clause, returning (City, error) with ErrNotFound on no match; add the handler and route the same way handleDeleteCity and DELETE /cities/{id} are wired in server.go/handlers.go.