Refactor reading_lists views to reduce duplication

[bpepple]

Summary

• Extract shared reading-list visibility filtering and stat annotations into a ReadingListQuerySet (visible_to() / with_list_stats()), replacing four near-identical copies across ReadingListListView, SearchReadingListListView, UserReadingListListView, ReadingListDetailView, and ReadingListItemsLoadMore.
• Add ReadingListFromSlugMixin / ManageReadingListMixin to remove duplicated slug-lookup dispatch(), test_func(), get_success_url(), and context-injection boilerplate from AssignReadingListToMetronView, RemoveIssueFromReadingListView, AddIssueWithAutocompleteView, AddIssuesFromSeriesView, and AddIssuesFromArcView.
• Extract filter_issues_by_number_range() and add_issues_to_reading_list() helpers, deduplicating the issue-range filtering and beginning/end reorder logic shared by AddIssuesFromSeriesView and AddIssuesFromArcView (and dropping their noqa: PLR0912, PLR0915 complexity suppressions).
• Extract the issue-type/series/publisher breakdown and featured-creators/top-characters computation out of ReadingListDetailView.get_context_data() into a standalone build_reading_list_breakdown_context() function.
• Update reading_lists/TECHNICAL.md to document the new queryset methods, mixins, and helper functions.

No behavior changes — this is a pure internal refactor for maintainability.