feat: add space weather image prefetch and stable cache-busting

Backend: Add /prefetch-images endpoint that warms the image cache in
parallel using a thread pool, skipping already-cached images.

Frontend: Trigger prefetch on mode init so images load instantly.
Replace per-request Date.now() cache-bust with a 5-minute rotating
key to allow browser caching aligned with backend max-age.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/space_weather.py

@@ -304,3 +304,36 @@ def get_image(key: str):
     _cache_set(cache_key, img_data, TTL_IMAGE)
     return Response(img_data, content_type=entry['content_type'],
                     headers={'Cache-Control': 'public, max-age=300'})
+
+
+@space_weather_bp.route('/prefetch-images')
+def prefetch_images():
+    """Warm the image cache by fetching all whitelisted images in parallel."""
+    # Only fetch images not already cached
+    to_fetch = {}
+    for key, entry in IMAGE_WHITELIST.items():
+        cache_key = f'img_{key}'
+        if _cache_get(cache_key) is None:
+            to_fetch[key] = entry
+
+    if not to_fetch:
+        return jsonify({'status': 'all cached', 'count': 0})
+
+    def _fetch_and_cache(key: str, entry: dict) -> bool:
+        img_data = _fetch_bytes(entry['url'])
+        if img_data:
+            _cache_set(f'img_{key}', img_data, TTL_IMAGE)
+            return True
+        return False
+
+    fetched = 0
+    with concurrent.futures.ThreadPoolExecutor(max_workers=6) as executor:
+        futures = {
+            executor.submit(_fetch_and_cache, k, e): k
+            for k, e in to_fetch.items()
+        }
+        for future in concurrent.futures.as_completed(futures):
+            if future.result():
+                fetched += 1
+
+    return jsonify({'status': 'ok', 'fetched': fetched, 'cached': len(IMAGE_WHITELIST) - len(to_fetch)})