feat(dashboard): delete conversation turns (per-turn ✕ + 전체 삭제)

Add a per-turn ✕ button on each 대화 카드 and a "🗑 전체 삭제" button in the turn
filter bar. Monitor gains delete_turn(id)/clear_turns() which broadcast
turn_deleted/turns_cleared; the page removes them live (all connected clients).
Routes: POST /api/turns/delete, POST /api/turns/clear.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

wsai/monitor.py

@@ -258,6 +258,26 @@ class Monitor:
             self._events.clear()
         self._broadcast({"type": "logs_cleared"})
 
+    def delete_turn(self, turn_id: int) -> bool:
+        """Remove one conversation turn by id (dashboard per-turn '✕')."""
+        found = False
+        with self._lock:
+            for t in list(self._turns):
+                if t.id == turn_id:
+                    self._turns.remove(t)
+                    found = True
+                    break
+        if found:
+            self._broadcast({"type": "turn_deleted", "id": turn_id})
+        return found
+
+    def clear_turns(self) -> None:
+        """Wipe all conversation turns (dashboard '대화 전체 삭제'). Broadcasts a
+        reset so every connected page clears its turn list too."""
+        with self._lock:
+            self._turns.clear()
+        self._broadcast({"type": "turns_cleared"})
+
     # -- turns ------------------------------------------------------------ #
     def turn(self, source: str = "voice") -> Turn:
         with self._lock: