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)})

static/js/modes/space-weather.js

@@ -19,6 +19,11 @@ const SpaceWeather = (function () {
     let _solarImageKey = 'sdo_193';
     let _drapFreq = 'drap_global';
 
+    /** Stable cache-bust key that rotates every 5 minutes (matches backend max-age). */
+    function _cacheBust() {
+        return 'v=' + Math.floor(Date.now() / 300000);
+    }
+
     // -------------------------------------------------------------------
     // Public API
     // -------------------------------------------------------------------
@@ -27,6 +32,8 @@ const SpaceWeather = (function () {
         if (!_initialized) {
             _initialized = true;
         }
+        // Warm the backend image cache in parallel before rendering
+        fetch('/space-weather/prefetch-images').catch(function () {});
         refresh();
         _startAutoRefresh();
     }
@@ -50,7 +57,7 @@ const SpaceWeather = (function () {
             const img = new Image();
             img.onload = function () { frame.innerHTML = ''; frame.appendChild(img); };
             img.onerror = function () { frame.innerHTML = '<div class="sw-empty">Failed to load image</div>'; };
-            img.src = '/space-weather/image/' + key + '?t=' + Date.now();
+            img.src = '/space-weather/image/' + key + '?' + _cacheBust();
             img.alt = key;
         }
     }
@@ -64,7 +71,7 @@ const SpaceWeather = (function () {
             const img = new Image();
             img.onload = function () { frame.innerHTML = ''; frame.appendChild(img); };
             img.onerror = function () { frame.innerHTML = '<div class="sw-empty">Failed to load image</div>'; };
-            img.src = '/space-weather/image/' + key + '?t=' + Date.now();
+            img.src = '/space-weather/image/' + key + '?' + _cacheBust();
             img.alt = key;
         }
     }