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/components/profile/WatchedListsSection.tsx

@@ -101,15 +101,14 @@ function WatchedSeriesCard({
       {/* Cover */}
       <button onClick={onNavigate} className="flex-shrink-0">
         <div className={`relative w-14 ${squareCovers ? 'aspect-square' : 'aspect-[2/3]'} rounded-lg overflow-hidden bg-gradient-to-br from-emerald-100 to-teal-200 dark:from-emerald-900 dark:to-teal-900`}>
-          {item.coverArtUrl ? (
-            <Image src={item.coverArtUrl} alt={item.seriesTitle} fill className="object-cover" sizes="56px" />
-          ) : (
-            <div className="absolute inset-0 flex items-center justify-center">
-              <svg className="w-6 h-6 text-emerald-400" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={1.5} d="M12 6.042A8.967 8.967 0 006 3.75c-1.052 0-2.062.18-3 .512v14.25A8.987 8.987 0 016 18c2.305 0 4.408.867 6 2.292m0-14.25a8.966 8.966 0 016-2.292c1.052 0 2.062.18 3 .512v14.25A8.987 8.987 0 0018 18a8.967 8.967 0 00-6 2.292m0-14.25v14.25" />
-              </svg>
-            </div>
-          )}
+          <Image
+            src={item.coverArtUrl || '/placeholder_cover.svg'}
+            alt={item.seriesTitle}
+            fill
+            className="object-cover"
+            sizes="56px"
+            onError={(e) => { (e.target as HTMLImageElement).src = '/placeholder_cover.svg'; }}
+          />
         </div>
       </button>