Add series-level relevance boost for unrated books

Books with 0 ratings now inherit 60% of their series' aggregate
relevance score instead of scoring flat zero. This ensures new
releases from popular series (e.g. Dungeon Crawler Carl) rank
meaningfully instead of being buried below obscure titles.

Series scores are computed once across all years using the same
Bayesian-weighted formula, then passed into per-year scoring.