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/assets/css/lobby.css

@@ -13,6 +13,22 @@
     margin-bottom: 2rem;
 }
 
+.teaching-box {
+    text-align: left;
+    margin-bottom: 2rem;
+    padding: 1rem 1.25rem;
+}
+
+.teaching-box .checkbox-label.disabled {
+    opacity: 0.6;
+    cursor: not-allowed;
+}
+
+.teaching-box .info-text {
+    margin: 0.5rem 0 0;
+    font-size: 0.85rem;
+}
+
 .link-item {
     margin-bottom: 1.5rem;
 }