fix re-extract: add heartbeat yield, batch index writes, handle HTTP errors in UI

- Generator now yields a 'status' event immediately so the client can
  distinguish 'working' from 'failed silently before first event'
- Batch mode: call write_activity per file but write index.json and
  athlete.json only once at the end (was O(n²) — 2015 rewrites)
- JS: check r.ok before reading the body stream; show HTTP error detail
  instead of staying stuck at 'Starting…'
- Handle 'status' event type in the progress log

site/src/pages/admin/index.astro

@@ -190,13 +190,15 @@ import Base from '../../layouts/Base.astro';
 
         let imported = 0, skipped = 0, errors = 0;
         try {
-          const es = new EventSource(`/api/admin/users/${h}/reextract-originals`);
-          // EventSource only does GET; use fetch + ReadableStream instead
-          es.close();
-
           const r = await fetch(`/api/admin/users/${h}/reextract-originals`, {
             method: 'POST', credentials: 'include',
           });
+          if (!r.ok) {
+            const errText = await r.text().catch(() => r.status.toString());
+            reextractSummary.textContent = `Error ${r.status}: ${errText}`;
+            reextractClose.disabled = false;
+            return;
+          }
           const reader = r.body!.getReader();
           const decoder = new TextDecoder();
           let buf = '';
@@ -210,7 +212,9 @@ import Base from '../../layouts/Base.astro';
               const dataLine = chunk.split('\n').find(l => l.startsWith('data: '));
               if (!dataLine) continue;
               const ev = JSON.parse(dataLine.slice(6));
-              if (ev.type === 'progress') {
+              if (ev.type === 'status') {
+                reextractSummary.textContent = ev.message;
+              } else if (ev.type === 'progress') {
                 const color = ev.status === 'imported' ? 'text-green-400'
                             : ev.status === 'error'    ? 'text-red-400'
                             : 'text-zinc-500';