Add dice-roll animation and tighten attack-roll prompt rules

Broadcast a "rolling" WS event when the DM invokes roll_dice, and show
a cycling dice-emoji animation with the roll notation in the chat while
waiting for the result, instead of just the generic typing indicator.

Also split the dice-roll prompt rule into skill/save checks (DC shown
to the player) vs. attack rolls (AC stays secret but must still be
labeled as a roll), and made the "always roll yourself, never ask the
player to roll" instruction unconditional.

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

backend/app/api/ws_game.py

@@ -120,8 +120,13 @@ async def game_websocket(websocket: WebSocket, game_id: uuid.UUID, ticket: str)
                     )
                     await manager.broadcast(game_id, {"type": "typing"})
 
+                    async def _broadcast_roll(notation: str) -> None:
+                        await manager.broadcast(game_id, {"type": "rolling", "notation": notation})
+
                     try:
-                        dm_message = await run_dm_turn(session, game_id, latest_player_message=content)
+                        dm_message = await run_dm_turn(
+                            session, game_id, latest_player_message=content, on_roll=_broadcast_roll
+                        )
                     except Exception:  # noqa: BLE001
                         logger.exception("DM turn failed for game %s", game_id)
                         await manager.broadcast(game_id, {"type": "error", "detail": "dm_turn_failed"})