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/admin/page.tsx

@@ -176,23 +176,13 @@ function PendingApprovalSection({ requests }: { requests: PendingApprovalRequest
                 <div className="flex gap-3">
                   {/* Cover Image */}
                   <div className="flex-shrink-0">
-                    {request.audiobook.coverArtUrl ? (
-                      <img
-                        src={request.audiobook.coverArtUrl}
-                        alt={request.audiobook.title}
-                        className="w-16 h-16 rounded object-cover"
-                      />
-                    ) : (
-                      <div className="w-16 h-16 rounded bg-gray-200 dark:bg-gray-700 flex items-center justify-center">
-                        <svg
-                          className="w-8 h-8 text-gray-400 dark:text-gray-600"
-                          fill="currentColor"
-                          viewBox="0 0 20 20"
-                        >
-                          <path d="M9 4.804A7.968 7.968 0 005.5 4c-1.255 0-2.443.29-3.5.804v10A7.969 7.969 0 015.5 14c1.669 0 3.218.51 4.5 1.385A7.962 7.962 0 0114.5 14c1.255 0 2.443.29 3.5.804v-10A7.968 7.968 0 0014.5 4c-1.255 0-2.443.29-3.5.804V12a1 1 0 11-2 0V4.804z" />
-                        </svg>
-                      </div>
-                    )}
+                    {/* eslint-disable-next-line @next/next/no-img-element */}
+                    <img
+                      src={request.audiobook.coverArtUrl || '/placeholder_cover.svg'}
+                      alt={request.audiobook.title}
+                      className="w-16 h-16 rounded object-cover"
+                      onError={(e) => { (e.target as HTMLImageElement).src = '/placeholder_cover.svg'; }}
+                    />
                   </div>
 
                   {/* Book Info */}