chore: 将后端移动到 api/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/prompt/utils.py

@@ -0,0 +1,45 @@
+from django.db.models import Count, Q
+from .models import Conversation
+
+
+def can_access_task(user, task_id):
+    """自由创作只有创作者本人（和超管）能对话，其他任务对所有登录用户开放。"""
+    from account.models import RoleChoices
+    from task.models import Creation
+
+    owner_id = (
+        Creation.objects.filter(pk=task_id).values_list("owner_id", flat=True).first()
+    )
+    if owner_id is None:
+        return True
+    return owner_id == user.id or user.role == RoleChoices.SUPER
+
+
+def get_active_conversation(user, task_id):
+    """Return the conversation with the most messages for this user+task, or None."""
+    return (
+        Conversation.objects.filter(user=user, task_id=task_id)
+        .annotate(msg_count=Count("messages"))
+        .order_by("-msg_count", "-created")
+        .first()
+    )
+
+
+def get_or_create_active_conversation(user, task_id):
+    conv = get_active_conversation(user, task_id)
+    if not conv:
+        conv = Conversation.objects.create(user=user, task_id=task_id)
+    return conv
+
+
+def get_preceding_user_message(asst_msg):
+    """Return the user message immediately preceding an assistant message in its conversation."""
+    return (
+        asst_msg.conversation.messages.filter(role="user")
+        .filter(
+            Q(created__lt=asst_msg.created)
+            | Q(created=asst_msg.created, id__lt=asst_msg.id)
+        )
+        .order_by("-created", "-id")
+        .first()
+    )