The MinIO cache key only encoded page number, so all filter combinations
hit the same cache entry and returned unfiltered results. Introduce
BrowseFilteredHTMLKey() that encodes sort/genre/status into the key
(e.g. novelfire.net/html/new-isekai-completed/page-1.html); falls back
to the original page-only key for default filters to preserve existing
cached pages.
Adds GET /api/search Go endpoint that queries PocketBase books by title/author
substring and fetches novelfire.net /search results via parseBrowsePage(),
merging results with local-first de-duplication. The browse page gains a search
input that navigates to ?q=; results show local vs remote counts and hide
pagination/filter controls while in search mode.