Export cross-year series.json, use it for series/author/narrator modals

Series modal now shows ALL books across all years instead of just the
currently loaded year. DCC now shows 8 titles instead of 3.

Backend: exports static/data/series.json (420 series, 924 books) grouping
all filtered books by series name, sorted by series number.

Frontend: loads series.json on first modal open (cached), replaces the
year-limited allLoadedBooks filter with cross-year lookups. Works for
series, author, and narrator modals.