Fetch artwork offered only over http for the https page (#90)

Through ipodderx.sdf1.net the page is https, the browser upgrades an http:// image to https,
and a host with no https, such as The Secret Cabal's CDN, answers nothing, so no artwork. On an
https page, the page now asks /api/art for those, and ipx fetches them. It only fetches an
address some feed or entry names as its artwork, and only an image, up to 5 MB, so the route
cannot be pointed at anything else on the network.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web.rs

@@ -64,6 +64,7 @@ pub fn router(state: WebState) -> Router {
         .route("/admin", get(admin_page))
         .route("/admin.js", get(admin_js))
         .route("/media/{id}", get(media))
+        .route("/api/art", get(art))
         .layer(middleware::from_fn_with_state(state.clone(), auth))
         // Signing in cannot require being signed in, so these sit outside the auth layer.
         .route("/login", get(login_page))
@@ -1676,6 +1677,38 @@ async fn media(
     }
 }
 
+#[derive(Deserialize)]
+struct ArtQuery {
+    u: String,
+}
+
+/// Artwork a feed names on plain http, fetched here for the https page. The browser upgrades an
+/// http image on an https page to https, and a host with no https, such as The Secret Cabal's
+/// CDN, then answers nothing (issue #90).
+async fn art(State(state): State<WebState>, Query(q): Query<ArtQuery>) -> Response {
+    const MAX: usize = 5 << 20;
+    if !q.u.starts_with("http://") || !state.ctx.db.names_image(&q.u).await.unwrap_or(false) {
+        return (StatusCode::NOT_FOUND, "no feed names that artwork").into_response();
+    }
+    let got = async {
+        let mut r = state.ctx.client.get(&q.u).send().await?.error_for_status()?;
+        let kind = r.headers().get(header::CONTENT_TYPE).and_then(|v| v.to_str().ok()).unwrap_or("").to_owned();
+        anyhow::ensure!(kind.starts_with("image/"), "not an image: {kind}");
+        let mut body = Vec::new();
+        while let Some(c) = r.chunk().await? {
+            body.extend_from_slice(&c);
+            anyhow::ensure!(body.len() <= MAX, "larger than {MAX} bytes");
+        }
+        anyhow::Ok((kind, body))
+    };
+    match got.await {
+        Ok((kind, body)) => {
+            ([(header::CONTENT_TYPE, kind), (header::CACHE_CONTROL, "private, max-age=86400".into())], body).into_response()
+        }
+        Err(e) => (StatusCode::BAD_GATEWAY, format!("{e:#}")).into_response(),
+    }
+}
+
 #[derive(Deserialize)]
 struct Position {
     secs: i64,