Multiple admins

Adds Player.is_admin (creator starts with it; backfilled by migration).
Admin-key-authenticated POST /admin/set-admin grants/revokes it from the
admin panel; the creator's privileges can't be revoked. Admin-gated
backend routes (dev mode, skip character creation) and frontend controls
(corner menu, lobby start, scene start, end story) now check is_admin
instead of is_creator, and admins get the admin_key in their state blob
so granted players can open the admin panel.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/UpkeepPhase.svelte

@@ -344,7 +344,7 @@
             {/if}
         {/if}
 
-        {#if state.player.is_creator && state.game.phase === 'between_scenes'}
+        {#if state.player.is_admin && state.game.phase === 'between_scenes'}
             <div class="end-story-box">
                 <p class="info-text" style="font-size: 0.85rem;">All good stories end. When the crew agrees the legend is complete:</p>
                 <button on:click={finishGame} class="btn btn-danger">🏴‍☠️ End the Story</button>