v2.5.4

feat(pdf): shrink output 20x via Qiniu imageView2 server-side resize

Intercept all image requests on img.cdn.guoshuyu.cn during PDF rendering
and rewrite them to '?imageView2/2/w/1400/q/75/format/jpg'. Qiniu CDN
returns resized + JPEG-recompressed images server-side (1.2MB PNG -> 60KB
JPEG, verified). Across the 2189 Qiniu images in the book this should
shrink the merged PDF from ~916 MB (v2.5.3) to ~50 MB without local CPU
cost and without slowing PDF generation.

Also adds --only=path1,path2 for spot-checking single articles locally,
and logs rewriteCount so the CI run shows how many requests were rewritten.