chore: add ESLint with no-floating-promises and no-console rules

Sets up ESLint 10 flat config with @typescript-eslint/no-floating-promises (error)
and no-console (warn, allow warn/error). Fixes all floating promise errors in
server routes, orchestrator reader cleanup, and frontend composables with void operator.
Game engine files get warning-level for intentional fire-and-forget async.

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

frontend/src/composables/useHumanChallenge.ts

@@ -51,7 +51,7 @@ export function useHumanChallenge(
           if (humanChoices.value.length > 0 && !humanAnswer.value.trim()) {
             humanAnswer.value = humanChoices.value[Math.floor(Math.random() * humanChoices.value.length)]
           }
-          submitHumanAnswer()
+          void submitHumanAnswer()
         }
       }
     }, 1000)
@@ -77,7 +77,7 @@ export function useHumanChallenge(
 
   function submitChoice(choice: string) {
     humanAnswer.value = choice
-    submitHumanAnswer()
+    void submitHumanAnswer()
   }
 
   async function pollForChallenge() {
@@ -110,8 +110,8 @@ export function useHumanChallenge(
 
   function startHumanPolling() {
     if (!myBotId.value) return
-    pollForChallenge()
-    humanPollHandle = setInterval(pollForChallenge, 400)
+    void pollForChallenge()
+    humanPollHandle = setInterval(() => { void pollForChallenge() }, 400)
   }
 
   function stopHumanPolling() {