feat: jc 引擎主线搬入 app/engine（315 测试基线全绿：320 去掉 admin 5 用例）

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

app/engine/providers/base.py

@@ -0,0 +1,24 @@
+from abc import ABC, abstractmethod
+from collections.abc import AsyncGenerator
+
+
+class AIProvider(ABC):
+    @abstractmethod
+    async def chat(self, messages: list[dict], system_prompt: str) -> str:
+        """Send messages and return assistant reply text."""
+
+    @abstractmethod
+    async def stream_chat(
+        self, messages: list[dict], system_prompt: str
+    ) -> AsyncGenerator[str, None]:
+        """Stream assistant reply token by token."""
+
+    @abstractmethod
+    async def chat_with_extraction(
+        self, messages: list[dict], system_prompt: str
+    ) -> tuple[str, dict]:
+        """Send messages and return (reply_text, extracted_json).
+
+        The provider should request JSON output and parse it.
+        Falls back to empty dict if parsing fails.
+        """