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/pyproject.toml

@@ -0,0 +1,22 @@
+[project]
+name = "webapi"
+version = "0.1.0"
+description = ""
+readme = "README.md"
+requires-python = ">=3.13"
+dependencies = [
+    "django>=6.0.1",
+    "django-cors-headers>=4.9.0",
+    "django-extensions>=4.1",
+    "django-ninja>=1.5.3",
+    "gunicorn>=24.1.1",
+    "psycopg[binary]>=3.3.2",
+    "pydantic[email]>=2.12.5",
+    "python-dotenv>=1.2.1",
+    "uvicorn[standard]>=0.40.0",
+    "django-redis>=6.0.0",
+    "redis>=7.1.0",
+    "channels[daphne]>=4.3.2",
+    "channels-redis>=4.3.0",
+    "openai>=2.24.0",
+]