fix: cache ISS position/schedule and parallelize SSTV init API calls

SSTV mode was slow to populate next-pass countdown and ISS location map
due to uncached skyfield computation and sequential JS API calls.

- Cache ISS position (10s TTL) and schedule (15min TTL, keyed by rounded lat/lon)
- Cache skyfield timescale object (expensive to create on every request)
- Reduce external API timeouts from 5s to 3s
- Fire checkStatus, loadImages, loadIssSchedule, updateIssPosition in parallel via Promise.all

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

static/js/modes/sstv.js

@@ -39,13 +39,23 @@ const SSTV = (function() {
      * Initialize the SSTV mode
      */
     function init() {
-        checkStatus();
-        loadImages();
+        // Load location inputs first (sync localStorage read needed for lat/lon params)
         loadLocationInputs();
-        loadIssSchedule();
+
+        // Fire all API calls in parallel — schedule is the slowest, don't let it block
+        Promise.all([
+            checkStatus(),
+            loadImages(),
+            loadIssSchedule(),
+            updateIssPosition(),
+        ]).catch(err => console.error('SSTV init error:', err));
+
+        // DOM-only setup (no network, fast)
         initMap();
-        startIssTracking();
         startCountdown();
+        // ISS tracking interval (first call already in Promise.all above)
+        if (issUpdateInterval) clearInterval(issUpdateInterval);
+        issUpdateInterval = setInterval(updateIssPosition, 5000);
         // Ensure Leaflet recomputes dimensions after the SSTV pane becomes visible.
         setTimeout(() => invalidateMap(), 80);
         setTimeout(() => invalidateMap(), 260);