fix: add ownership check and 404 responses to ai-sessions endpoints

Cross-tenant isolation audit found:
- retry-psa-push had NO ownership check (CRITICAL) — any user could retry any session's PSA push
- save_task_lane used db.get() without ownership filter, returned 403 revealing existence
- get_session returned 403 instead of 404 for unauthorized access
- stream_documentation returned 403 instead of 404

All now use query-level user_id filtering and return 404 to avoid revealing existence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -519,11 +519,15 @@ async def save_task_lane(
     _: None = Depends(require_engineer_or_admin),
 ):
     """Save the current task lane state including user's in-progress responses."""
-    session = await db.get(AISession, session_id)
+    result = await db.execute(
+        select(AISession).where(
+            AISession.id == session_id,
+            AISession.user_id == current_user.id,
+        )
+    )
+    session = result.scalar_one_or_none()
     if not session:
         raise HTTPException(status_code=404, detail="Session not found")
-    if session.user_id != current_user.id:
-        raise HTTPException(status_code=403, detail="Not your session")
 
     payload = {
         "questions": [q.model_dump() for q in body.questions],
@@ -901,7 +905,7 @@ async def get_session(
     pkg = session.escalation_package or {}
     is_handler = pkg.get("picked_up_by") == str(current_user.id)
     if session.user_id != current_user.id and session.escalated_to_id != current_user.id and not is_handler:
-        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Not authorized")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Session not found")
 
     return _build_session_detail(session)
 
@@ -942,13 +946,14 @@ async def stream_documentation(
 
     # Verify session ownership
     result = await db.execute(
-        select(AISession).where(AISession.id == session_id)
+        select(AISession).where(
+            AISession.id == session_id,
+            AISession.user_id == current_user.id,
+        )
     )
     session = result.scalar_one_or_none()
     if not session:
         raise HTTPException(status_code=404, detail="Session not found")
-    if session.user_id != current_user.id:
-        raise HTTPException(status_code=403, detail="Not authorized")
 
     async def event_generator():
         try:
@@ -1043,6 +1048,19 @@ async def retry_psa_push_endpoint(
     """Manually retry a failed PSA documentation push."""
     from app.models.psa_post_log import PsaPostLog
 
+    # Verify the session belongs to the current user
+    session_result = await db.execute(
+        select(AISession).where(
+            AISession.id == session_id,
+            AISession.user_id == current_user.id,
+        )
+    )
+    if not session_result.scalar_one_or_none():
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Session not found",
+        )
+
     # Find the latest failed push log for this session
     result = await db.execute(
         select(PsaPostLog)