Add per-user home sections & unified Audible cache

Introduce per-user configurable home page sections and a unified Audible cache/category model. Adds Prisma models (UserHomeSection, AudibleCacheCategory) and migrations to create tables and remove legacy popular/new_release flags; updates schema.prisma accordingly. Add API routes for user home sections, live Audible categories, and category-based audiobook listing, and refactor popular/new-releases/covers routes to read from AudibleCacheCategory. Frontend: new HomeSection component, HomeSectionConfigModal, useHomeSections hook, and homepage changes to render dynamic sections plus image fallback to a placeholder SVG. Also add placeholder_cover.svg and tests for home sections and the audible refresh processor.

src/app/login/page.tsx

@@ -486,6 +486,7 @@ function LoginContent() {
                       quality={70}
                       priority={index < 10}
                       loading={index < 10 ? 'eager' : 'lazy'}
+                      onError={(e) => { (e.target as HTMLImageElement).src = '/placeholder_cover.svg'; }}
                     />
                     <div className="absolute inset-0 bg-gradient-to-t from-black/40 to-transparent" />
                   </div>