Redirect large/non-inline lesson files to CDN instead of proxying

ZIP/DOCX etc. download natively — no need to force attachment; and the
157 MB Second Brain Vault archive must stream from Caddy with Range,
not through the app. Proxy attachment stays for small inline types (PDF/images).

src/app/api/lesson-files/[id]/download/route.ts

@@ -29,6 +29,17 @@ function extFromUrl(url: string): string {
   return dot >= 0 ? clean.slice(dot + 1).toLowerCase() : "";
 }
 
+// Types a browser renders inline (so the direct link "opens in a tab" instead of
+// downloading). Only these need proxying with a forced attachment header. ZIP,
+// DOCX, etc. already download natively → redirect straight to the CDN.
+const INLINE_EXT = new Set([
+  "pdf", "png", "jpg", "jpeg", "webp", "gif", "svg", "avif",
+  "txt", "htm", "html", "mp4", "mov", "webm", "mp3", "wav",
+]);
+// Above this, don't proxy through the app — redirect so the file streams from
+// the CDN/Caddy with Range support (e.g. the 157 MB Second Brain Vault archive).
+const PROXY_MAX_BYTES = 50 * 1024 * 1024;
+
 export async function GET(
   _req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
@@ -59,12 +70,26 @@ export async function GET(
     }
   }
 
+  const ext = extFromUrl(file.url);
+
+  // Non-inline types download natively — send the browser straight to the CDN
+  // (native download, Range/resume, no app load). Auth is already enforced above.
+  if (!INLINE_EXT.has(ext)) {
+    return NextResponse.redirect(file.url, 302);
+  }
+
   const upstream = await fetch(file.url);
   if (!upstream.ok || !upstream.body) {
     return NextResponse.json({ error: "file_unavailable" }, { status: 502 });
   }
 
-  const ext = extFromUrl(file.url);
+  // Guard: a very large inline file still shouldn't stream through the app.
+  const upstreamLen = Number(upstream.headers.get("content-length") ?? "0");
+  if (upstreamLen > PROXY_MAX_BYTES) {
+    await upstream.body.cancel();
+    return NextResponse.redirect(file.url, 302);
+  }
+
   const contentType =
     upstream.headers.get("content-type") ??
     CONTENT_TYPES[ext] ??