feat(app): add POST /grade endpoint with active-episode guard and programmatic grader

server/app.py

@@ -64,3 +64,17 @@ async def get_state():
 @app.get("/history")
 async def get_history():
     return {"history": env.get_history()}
+
+@app.post("/grade")
+async def grade_episode():
+    if env.is_active:
+        raise HTTPException(status_code=400, detail="Episode still active — finish it before grading.")
+
+    history = env.get_history()
+    if not history:
+        raise HTTPException(status_code=400, detail="No episode history to grade.")
+
+    from graders.programmatic_grader import grade_episode as do_grade
+    result = do_grade(history)
+    result["episode_id"] = env.episode_id
+    return result