feat(importer): _supersede() now applies the new (larger) file's sidecar — operator wanted to scan GS download dir to supersede smaller IR-migrated images AND wire up gallery-dl Post metadata, but supersede was file-only and silently dropped the sidecar.

_apply_sidecar is additive: it find-or-creates Post/Source/ImageProvenance
and sets primary_post_id NULL-only, so any IR-migration provenance on the
existing row survives untouched and the new GS sidecar adds a second
ImageProvenance pointing at the freshly-created Post.

Wrapped in try/except so a malformed sidecar can't unwind the file-swap
commit — the file replacement is the critical operation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/importer.py

@@ -722,6 +722,15 @@ class Importer:
         row id (so tags/series/curation stay attached). ML is cleared so
         the import task re-derives it on the new pixels.
 
+        After the file swap, the new file's adjacent gallery-dl sidecar
+        (if any) is applied via _apply_sidecar — operator-flagged
+        2026-05-25: scanning a GS download dir with smaller IR-migrated
+        images on the receiving end used to swap files but lose the GS
+        sidecar's post metadata entirely. _apply_sidecar is additive
+        (find-or-create Post / Source / ImageProvenance, NULL-only
+        primary_post_id update) so any pre-existing Post linkage
+        survives untouched.
+
         If `new_path` is provided, `source` is assumed to ALREADY be at
         that path (FC-3c attach_in_place case) — skip the copy step.
         Otherwise the file is copied via _copy_to_library."""
@@ -751,6 +760,22 @@ class Importer:
         self.session.flush()
         self.session.commit()
 
+        # Sidecar enrichment from the new (larger) file's location.
+        # _apply_sidecar resolves artist from the sidecar itself if the
+        # existing row has none, and is internally guarded against
+        # missing-or-malformed sidecars (silent return).
+        try:
+            self._apply_sidecar(existing, source, None)
+        except Exception as exc:
+            # Don't unwind the supersede DB swap if sidecar parsing
+            # blows up unexpectedly — the file replacement is the
+            # critical operation, sidecar is enrichment.
+            log.warning(
+                "sidecar enrichment failed during supersede of "
+                "image_record.id=%s from %s: %s",
+                existing.id, source, exc,
+            )
+
         for stale in (old_path, old_thumb):
             if not stale or stale == str(dest):
                 # If the supersede kept the file in place (new_path == old