fix: add payload size limits to task lane save endpoint

- Max 50 questions, 50 actions, 100 responses (Pydantic max_length)
- Max 256KB total serialized payload (prevents DB bloat)
- Existing guards: JWT auth, role check, ownership check, rate limit

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -517,11 +517,18 @@ async def save_task_lane(
     if session.user_id != current_user.id:
         raise HTTPException(status_code=403, detail="Not your session")
 
-    session.pending_task_lane = {
+    payload = {
         "questions": [q.model_dump() for q in body.questions],
         "actions": [a.model_dump() for a in body.actions],
         "responses": body.responses,
     }
+
+    # Guard against oversized payloads (max 256KB serialized)
+    import json
+    if len(json.dumps(payload)) > 256 * 1024:
+        raise HTTPException(status_code=413, detail="Task lane payload too large")
+
+    session.pending_task_lane = payload
     await db.commit()
 
 

backend/app/schemas/ai_session.py

@@ -289,10 +289,11 @@ class ChatMessageResponse(BaseModel):
 
 class SaveTaskLaneRequest(BaseModel):
     """Save the full task lane state (AI items + user responses)."""
-    questions: list[QuestionItem] = []
-    actions: list[ActionItem] = []
+    questions: list[QuestionItem] = Field(default_factory=list, max_length=50)
+    actions: list[ActionItem] = Field(default_factory=list, max_length=50)
     responses: list[dict[str, Any]] = Field(
         default_factory=list,
+        max_length=100,
         description="User's in-progress task responses with state/value",
     )