fix(import): resolve artist from path for enrich-on-duplicate (#718)

The dedup branches of _import_media linked the existing image to the new post via
_apply_sidecar(artist=None), relying on the SIDECAR to carry the artist. But an
archive member's artist comes from its path, and under post-first the per-media
sidecar is minimal (no artist) — so a re-packed / cross-posted archive image
deduped and was left UNLINKED from the new post, i.e. the post showed "no images".

Resolve the path-anchored artist (derive_top_level_artist) up-front in
_import_media and pass it to both enrich-on-duplicate branches (sha256 + phash
larger_exists) and the new-record path. Drop the now-dead _attach_artist helper
(its logic is inlined at the single new-record call site).

Surfaced by the new test_archive_all_deduped_is_benign_not_flagged (was asserting
2==4: the second post got no provenance links).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/importer.py

@@ -682,6 +682,15 @@ class Importer:
                         error=f"{pct:.2%} transparent",
                     )
 
+        # Artist anchored to the attribution path (folder→artist), resolved
+        # UP-FRONT so the enrich-on-duplicate branches link provenance with the
+        # right artist even when the sidecar carries none — which is now the norm
+        # for archive members under post-first (the per-media sidecar is minimal).
+        # Without this a cross-posted / re-packed archive image deduped and was
+        # left UNLINKED from the new post = a post showing "no images" (#718).
+        _artist_name = derive_top_level_artist(attribution_path, self.import_root)
+        path_artist = self._upsert_artist(_artist_name) if _artist_name else None
+
         # Hash dedup (exact).
         sha = _sha256_of(source)
         existing_stmt = select(ImageRecord).where(ImageRecord.sha256 == sha)
@@ -691,9 +700,9 @@ class Importer:
                 # Enrich-on-duplicate (parity with attach_in_place): a re-scanned
                 # file that is a byte-dup of an existing image still links its
                 # post via _apply_sidecar, so a cross-posted image shows on every
-                # post. Artist is derived from the sidecar here (not yet resolved).
+                # post.
                 self._apply_sidecar(
-                    existing, attribution_path, None,
+                    existing, attribution_path, path_artist,
                     explicit_source=explicit_source,
                 )
                 self.session.commit()
@@ -727,7 +736,7 @@ class Importer:
                     larger = self.session.get(ImageRecord, match_id)
                     if larger is not None:
                         self._apply_sidecar(
-                            larger, attribution_path, None,
+                            larger, attribution_path, path_artist,
                             explicit_source=explicit_source,
                         )
                         self.session.commit()
@@ -761,13 +770,11 @@ class Importer:
         self.session.flush()
         self._phash_cache_append(phash, width, height, record.id)
 
-        # Folder→artist (anchored to attribution_path).
-        artist = None
-        artist_name = derive_top_level_artist(
-            attribution_path, self.import_root
-        )
-        if artist_name:
-            artist = self._attach_artist(record, artist_name)
+        # Folder→artist (path_artist resolved up-front above); bind it to the
+        # new record so it carries the canonical Artist.
+        if path_artist is not None and record.artist_id is None:
+            record.artist_id = path_artist.id
+            self.session.flush()
 
         # Sidecar provenance (best-effort; never fails the import).
         # explicit_source lets the FC-3c download path bind the new
@@ -777,7 +784,7 @@ class Importer:
         # subscription-downloaded zip previously lost subscription
         # linkage if the on-disk layout didn't match assumptions.
         self._apply_sidecar(
-            record, attribution_path, artist, explicit_source=explicit_source,
+            record, attribution_path, path_artist, explicit_source=explicit_source,
         )
 
         # Thumbnail is queued separately by the calling task; the importer
@@ -1076,17 +1083,6 @@ class Importer:
             self.session.flush()
         return artist
 
-    def _attach_artist(self, record: ImageRecord, artist_name: str) -> Artist:
-        """Upsert the Artist and set it as the image's canonical artist.
-        Artist-kind tags were retired in FC-2d-vii-c — the Artist row is
-        the single source of truth. Returns the Artist (sidecar
-        provenance attaches its Source to it)."""
-        artist = self._upsert_artist(artist_name)
-        if record.artist_id is None:
-            record.artist_id = artist.id
-            self.session.flush()
-        return artist
-
     @staticmethod
     def _sidecar_artist_name(data: dict) -> str | None:
         for k in ("artist", "author_name"):