fix: track and clear all intervals and event listeners

- FightPage.vue: store SSE listener refs in array, removeEventListener
  on each before close() in disconnectSSE()
- FightScene.ts: add cleanupTimers Set with trackedInterval/trackedTimeout
  helpers; replace key setInterval calls (projectile trails, entrance
  flames, talking animation, exhaust effects, dimensional shift, hole
  fade) with tracked versions; clear all on scene destroy()
- rate-limit.ts: export cleanupInterval handle for graceful shutdown

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

server/src/middleware/rate-limit.ts

@@ -5,7 +5,7 @@ const isDev = process.env.NODE_ENV !== 'production'
 const hitCounts = new Map<string, { count: number; resetAt: number }>()
 
 // Cleanup stale entries every 5 minutes
-setInterval(() => {
+export const cleanupInterval = setInterval(() => {
   const now = Date.now()
   for (const [key, entry] of hitCounts) {
     if (now > entry.resetAt) hitCounts.delete(key)