refactor stage 1

detect/checkpoint/__init__.py

@@ -1,14 +1,18 @@
 """
-Stage checkpoint, replay, and retry.
+Checkpoint system — Timeline + Checkpoint tree.
 
     detect/checkpoint/
         frames.py      — frame image S3 upload/download
-        serializer.py  — state ↔ JSON conversion
-        storage.py     — checkpoint save/load/list (Postgres + S3)
-        replay.py      — replay_from, OverrideProfile
+        storage.py     — Timeline + Checkpoint (Postgres + MinIO)
+        replay.py      — replay (TODO: migrate to new model)
         tasks.py       — retry_candidates Celery task
 """
 
-from .storage import save_checkpoint, load_checkpoint, list_checkpoints
+from .storage import (
+    create_timeline,
+    get_timeline_frames,
+    get_timeline_frames_b64,
+    save_stage_output,
+    load_stage_output,
+)
 from .frames import save_frames, load_frames
-from .replay import replay_from, OverrideProfile