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/tests/api_mangas_cover.rs

@@ -1,6 +1,7 @@
 mod common;
 
 use axum::http::StatusCode;
+use http_body_util::BodyExt;
 use serde_json::{json, Value};
 use sqlx::PgPool;
 use tower::ServiceExt;
@@ -45,6 +46,65 @@ fn cover_form(bytes: &[u8]) -> MultipartBuilder {
     MultipartBuilder::new().add_file("cover", "cover.bin", "application/octet-stream", bytes)
 }
 
+/// A real, decodable PNG (unlike `fake_png_bytes`, which is only magic bytes)
+/// so the thumbnail endpoint has something to resize.
+fn real_png(width: u32, height: u32) -> Vec<u8> {
+    let mut buf = std::io::Cursor::new(Vec::new());
+    image::RgbImage::from_pixel(width, height, image::Rgb([10, 120, 200]))
+        .write_to(&mut buf, image::ImageFormat::Png)
+        .unwrap();
+    buf.into_inner()
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn files_serves_downscaled_thumbnail_variant(pool: PgPool) {
+    let h = harness(pool);
+    let (_, cookie) = register_user(&h.app).await;
+    let manga = create_manga_with_cover(&h.app, &cookie, "Thumb", None).await;
+    let id = id_of(&manga);
+
+    // Upload a real 800x400 PNG cover.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(put_multipart_with_cookie(
+            &format!("/api/v1/mangas/{id}/cover"),
+            cover_form(&real_png(800, 400)),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let key = format!("mangas/{id}/cover.png");
+
+    // ?w=320 serves a 320px-wide variant with the source content-type.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get(&format!("/api/v1/files/{key}?w=320")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    assert_eq!(
+        resp.headers().get("content-type").unwrap().to_str().unwrap(),
+        "image/png"
+    );
+    let body = resp.into_body().collect().await.unwrap().to_bytes();
+    let thumb = image::load_from_memory(&body).unwrap();
+    assert_eq!(thumb.width(), 320, "served a 320px-wide variant");
+    assert_eq!(thumb.height(), 160, "aspect ratio preserved");
+
+    // The un-parametrised request still serves the full-resolution original.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get(&format!("/api/v1/files/{key}")))
+        .await
+        .unwrap();
+    let full = resp.into_body().collect().await.unwrap().to_bytes();
+    assert_eq!(image::load_from_memory(&full).unwrap().width(), 800);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn put_cover_sets_path_when_none_existed(pool: PgPool) {
     let h = harness(pool);