US07-04: Prove Concurrency and Crash Recovery (#86)

photo_pipeline/services/exif_checkpoint.py

@@ -25,6 +25,7 @@ import uuid
 from dataclasses import dataclass
 from datetime import datetime, timezone
 
+from photo_pipeline.faults import EXIF_WRITTEN, maybe_fault
 from photo_pipeline.integrations import exiftool
 from photo_pipeline.models import ExifProjection
 from photo_pipeline.services import hashing
@@ -120,6 +121,10 @@ def run(
     if not exiftool.apply_keywords(path, add=add, remove=remove):
         return CheckpointResult(FAILED, reason="write_failed")
 
+    # The file on disk has changed; nothing about it is recorded yet. A crash here
+    # is the worst case for metadata, so it is a fault control point (US07-04).
+    maybe_fault(EXIF_WRITTEN)
+
     after = exiftool.read_all(path)
     if after is None:
         return CheckpointResult(FAILED, reason="readback_unreadable")