Add teaching mode: admin can seed themselves as scene-1 Deep

An admin can now opt into "teaching mode" from the lobby, seeding
themselves as the Rank-3 player who must play the Deep in the first
scene (instead of leaving it to the random starting-rank roll), so they
can run the table for new crews.

- Game.teaching_deep_player_id (+ Alembic migration)
- maybe_finish_assign_techniques honors it before rolling ranks, so both
  the normal flow and the dev-mode skip path respect it
- Admin-gated POST .../teaching-mode toggle (pre-rank phases only)
- Lobby checkbox UI, greyed out if another admin already volunteered

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/LobbyPhase.svelte

@@ -24,6 +24,19 @@
             starting = false;
         }
     }
+
+    $: teaching = state.game.teaching_deep_player_id === state.player.id;
+    $: teachingTakenByOther = state.game.teaching_deep_player_id && !teaching;
+
+    async function toggleTeaching() {
+        try {
+            await apiRequest(`/game/${state.game.id}/player/${state.player.id}/teaching-mode`, 'POST', {
+                enabled: !teaching
+            });
+        } catch (err) {
+            console.error("Failed to toggle teaching mode:", err);
+        }
+    }
 </script>
 
 <div class="lobby-view text-center">
@@ -72,6 +85,22 @@
         {/if}
     </div>
     
+    {#if state.player.is_admin}
+    <div class="teaching-box glass-panel">
+        <label class="checkbox-label" class:disabled={teachingTakenByOther}>
+            <input type="checkbox" checked={teaching} disabled={teachingTakenByOther} on:change={toggleTeaching}>
+            <span>📚 <strong>Teaching mode:</strong> I'll be the Deep for the first scene</span>
+        </label>
+        <p class="info-text">
+            {#if teachingTakenByOther}
+                Another admin has already volunteered to teach as the Deep.
+            {:else}
+                Seeds you as the Rank-3 player who must play the Deep in scene 1, so you can run the table for new crews. Otherwise it's assigned at random.
+            {/if}
+        </p>
+    </div>
+    {/if}
+
     <div class="lobby-action">
         {#if state.player.is_admin}
             {#if state.players.length >= 3 || state.game.dev_mode}