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>

backend/src/api/mangas.rs

@@ -414,8 +414,14 @@ async fn put_cover(
                 Ok(()) | Err(StorageError::NotFound) => {}
                 Err(e) => return Err(e.into()),
             }
+            // Old key's cached thumbnails are now orphaned.
+            crate::api::files::purge_thumbnails(state.storage.as_ref(), prev).await;
         }
     }
+    // Cover keys are reused (mangas/{id}/cover.{ext}), so a same-extension
+    // replacement overwrites the blob at an existing key — drop any thumbnails
+    // cached for it so we don't serve a stale variant of the old cover.
+    crate::api::files::purge_thumbnails(state.storage.as_ref(), &new_key).await;
 
     repo::manga::set_cover_image_path(&state.db, id, &new_key, img.bytes.len() as i64).await?;
     Ok(Json(repo::manga::get_detail(&state.db, id).await?))
@@ -439,6 +445,7 @@ async fn delete_cover(
             Ok(()) | Err(StorageError::NotFound) => {}
             Err(e) => return Err(e.into()),
         }
+        crate::api::files::purge_thumbnails(state.storage.as_ref(), &key).await;
         repo::manga::clear_cover_image_path(&state.db, id).await?;
     }
     Ok(Json(repo::manga::get_detail(&state.db, id).await?))