feat: serve width-bounded thumbnail variants for image grids

Add /files/{key}?w=<px>: the endpoint decodes JPEG/PNG sources, downscales to
a width snapped to a small allow-list (aspect-preserving, never upscaling),
caches the result under a thumbs/ prefix, and serves it — so cover grids
download ~KB instead of the 1–5 MB original. Cover handlers purge cached
variants when a cover (whose key is reused) is replaced or deleted. Frontend
grids use new thumbUrl/thumbSrcset helpers (MangaCard with responsive srcset).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/BookmarkList.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-    import { fileUrl } from '$lib/api/client';
+    import { thumbUrl } from '$lib/api/client';
     import type { Bookmark } from '$lib/api/bookmarks';
     import { overflowTooltip } from '$lib/actions/overflowTooltip';
     import BookImage from '@lucide/svelte/icons/book-image';
@@ -19,7 +19,7 @@
             <a href="/manga/{b.manga_id}" class="cover-link" aria-hidden="true" tabindex="-1">
                 {#if b.manga_cover_image_path}
                     <img
-                        src={fileUrl(b.manga_cover_image_path)}
+                        src={thumbUrl(b.manga_cover_image_path, 320)}
                         alt=""
                         class="cover"
                         loading="lazy"