refactor: extract internal500 penalty logic to dedicated file

Move constants, detection, and penalty functions from
antigravity_gateway_service.go to antigravity_internal500_penalty.go.
Fix gofmt alignment and replace hardcoded duration strings with
constant references.

backend/internal/repository/internal500_counter_cache.go

@@ -9,7 +9,7 @@ import (
 )
 
 const (
-	internal500CounterPrefix    = "internal500_count:account:"
+	internal500CounterPrefix     = "internal500_count:account:"
 	internal500CounterTTLSeconds = 86400 // 24 小时兜底
 )