Add tracking system to support optional PostHog tracking in AI engine (#6040)

Co-authored-by: ConnorYoh <[email protected]>

engine/src/stirling/api/middleware.py

@@ -0,0 +1,23 @@
+from __future__ import annotations
+
+from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
+from starlette.requests import Request
+from starlette.responses import Response
+
+from stirling.services.tracking import current_user_id
+
+_USER_ID_HEADER = "X-User-Id"
+
+
+class UserIdMiddleware(BaseHTTPMiddleware):
+    """Extract X-User-Id header and set it as the current user for PostHog tracking."""
+
+    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
+        user_id = request.headers.get(_USER_ID_HEADER)
+        if user_id:
+            token = current_user_id.set(user_id)
+            try:
+                return await call_next(request)
+            finally:
+                current_user_id.reset(token)
+        return await call_next(request)