perf: leaderboard SWR cache + threadpool fix for season-III stalls

- Fix 1: UV_THREADPOOL_SIZE=24 via start_server.sh wrapper (libuv reads OS
  environ; process.env and PM2 env blocks don't propagate on this system)
- Fix 2: Stale-while-revalidate for leaderboards — serve cached/stale data
  instantly, refresh in background; dedicated aggregateCache isolated from
  the 100-entry responseCache; single-flight dedup for concurrent computes
- Fix 3: Background warmer precomputes current + last-completed season
  leaderboards at +20s boot and every 4 min
- Fix 5: Adaptive TTL (5 min live, 24 h completed) via aggregateCacheTtl
- Fixes 1+2 combined: player page stall 95s -> 3.6s under concurrent heavy
  leaderboard load; warm hits served in 1-4ms (was 13-53s)

web/server.js

@@ -509,31 +509,30 @@ async function getCachedData(type) {
     return cache.data;
 }
 
-// Initialize cache on startup — staggered to avoid overloading SQLite
+// Initialize only lightweight caches on the primary worker.
+// Player and vehicle leaderboards are too expensive to refresh in the background:
+// recent production runs took 140s+ and kept the API CPU saturated after callers timed out.
 async function initializeCache() {
-    log.info('[CACHE] Initializing leaderboard cache (staggered)...');
-    // Stats is lightest, do it first
+    if (!IS_PRIMARY_WORKER) return;
+
+    log.info('[CACHE] Initializing lightweight leaderboard cache (staggered)...');
     await updateCache('stats');
     log.info('[CACHE] Stats cache ready');
-    // Squadrons next
     await updateCache('squadrons');
     log.info('[CACHE] Squadrons cache ready');
-    // Players is heaviest
-    await updateCache('players');
-    log.info('[CACHE] Players cache ready');
-    // Vehicles last
-    await updateCache('vehicles');
-    log.info('[CACHE] Vehicles cache ready — all caches populated!');
 }
 
-// Auto-refresh caches staggered to avoid hammering the API all at once
-const cacheTypes = ['players', 'vehicles', 'stats', 'squadrons'];
-cacheTypes.forEach((type, i) => {
-    setInterval(async () => {
-        log.debug(`[CACHE] Auto-refreshing ${type} cache...`);
-        await updateCache(type).catch(log.error);
-    }, CACHE_DURATION + i * 15000); // stagger each by 15s
-});
+// Auto-refresh only lightweight caches from the primary worker. Heavy caches are
+// refreshed lazily by request so background work cannot pin SQLite indefinitely.
+if (IS_PRIMARY_WORKER) {
+    const cacheTypes = ['stats', 'squadrons'];
+    cacheTypes.forEach((type, i) => {
+        setInterval(async () => {
+            log.debug(`[CACHE] Auto-refreshing ${type} cache...`);
+            await updateCache(type).catch(log.error);
+        }, CACHE_DURATION + i * 15000);
+    });
+}
 
 // Log search cache statistics every 10 minutes
 setInterval(() => {