feat(tournament): enable external participation in tournaments

- Updated tournamentService to allow external participants by setting allowsExternal to true.
- Adjusted frontend TournamentTab component to reflect the change, enabling external participation for mini championships.

frontend/src/views/TournamentsView.vue

@@ -37,7 +37,7 @@
         <TournamentTab :allowsExternal="true" />
       </div>
       <div v-else-if="activeTab === 'mini'">
-        <TournamentTab :allowsExternal="false" :isMiniChampionship="true" />
+        <TournamentTab :allowsExternal="true" :isMiniChampionship="true" />
       </div>
       <div v-else-if="activeTab === 'official'">
         <OfficialTournaments />