fix(importer): guard _stamp_member_archive against a sidecar-less archive

A filesystem-imported archive with no adjacent sidecar has no Post, so
_post_for_sidecar returns None — and the milestone-#87 stamp call dereferenced
post.id. _stamp_member_archive already no-ops on a None post_id (no post → no
provenance to stamp); pass None instead of crashing. Caught by the existing
test_reimport_archive_is_idempotent (no-sidecar zip).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/importer.py

@@ -534,7 +534,9 @@ class Importer:
         # branches stay untouched. NULL-only so a re-extract never re-stamps and
         # the backfill (reextract task → this same path) is idempotent. Nested
         # members link to this OUTER archive — the only one stored as a blob.
-        self._stamp_member_archive(post.id, source, member_record_ids)
+        self._stamp_member_archive(
+            post.id if post is not None else None, source, member_record_ids,
+        )
         if member_ids:
             return ImportResult(
                 status="imported", image_id=member_ids[0],