feat: show rejoin link when bot is already in a fight

- Track active fight IDs per bot in orchestrator (Set → Map)
- Return fightId in "already in fight" error responses (409)
- Frontend shows "REJOIN FIGHT" link instead of generic error

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

server/src/routes/queue.ts

@@ -31,9 +31,10 @@ queueRouter.post('/join/:botId', async (c) => {
   try {
     const fightId = await joinQueue(botId)
     return c.json({ fightId, message: 'Matched! Fight starting.' })
-  } catch (err) {
+  } catch (err: any) {
     const message = err instanceof Error ? err.message : 'Queue error'
-    return c.json({ error: message }, 500)
+    const status = message.includes('already in a fight') ? 409 : 500
+    return c.json({ error: message, fightId: err?.fightId || undefined }, status)
   }
 })