Fix image uploads and reduce flyer request sizes

app/templates/new_concert.html

@@ -877,6 +877,48 @@ flyerInput.addEventListener(
     }
 );
 
+// Große Flyer vor dem Absenden verkleinern. Dadurch bleibt der Multipart-
+// Request unter typischen Proxy-Limits und der Server muss keine riesigen
+// Originaldateien verarbeiten.
+const concertForm = flyerInput.form;
+async function optimizeFlyerUpload(file) {
+    const bitmap = await createImageBitmap(file);
+    const maxSide = 1800;
+    const scale = Math.min(1, maxSide / Math.max(bitmap.width, bitmap.height));
+    const canvas = document.createElement("canvas");
+    canvas.width = Math.max(1, Math.round(bitmap.width * scale));
+    canvas.height = Math.max(1, Math.round(bitmap.height * scale));
+    const context = canvas.getContext("2d");
+    context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
+    bitmap.close();
+    let quality = 0.86;
+    let blob;
+    do {
+        blob = await new Promise(resolve => canvas.toBlob(resolve, "image/jpeg", quality));
+        quality -= 0.08;
+    } while (blob && blob.size > 900 * 1024 && quality >= 0.5);
+    if (!blob) throw new Error("Bild konnte nicht verarbeitet werden");
+    return new File([blob], "flyer.jpg", {type: "image/jpeg"});
+}
+
+concertForm.addEventListener("submit", async event => {
+    if (concertForm.dataset.flyerOptimized === "true") return;
+    const file = flyerInput.files[0];
+    if (!file) return;
+    event.preventDefault();
+    try {
+        const transfer = new DataTransfer();
+        transfer.items.add(await optimizeFlyerUpload(file));
+        flyerInput.files = transfer.files;
+        concertForm.dataset.flyerOptimized = "true";
+        concertForm.requestSubmit();
+    } catch (error) {
+        console.error("Flyer optimization failed:", error);
+        concertForm.dataset.flyerOptimized = "true";
+        concertForm.requestSubmit();
+    }
+});
+
 
 /* ============================================================
    HTML ESCAPING