refactor(importer): single _apply_post_fields predicate for both ingest paths (#842/#753)

The per-media path (_apply_sidecar) and the post-record path (upsert_post_record)
each carried a VERBATIM copy of the post-field write (url/title/date/description/
attachment_count/raw_metadata + external-link sync). Two copies of one concept =
the divergence risk #753 targets. Consolidate into one _apply_post_fields(post,
sd) helper both call — a single predicate for how a post body/links get stored,
so the two sources can't drift. Behavior identical (fill-with-non-empty); both
paths already covered by existing importer tests.

Groundwork for the planned post-first ingest model (single authoritative post
record; media attaches to it) as more platforms move onto the native ingester.

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

backend/app/services/importer.py

@@ -739,6 +739,29 @@ class Importer:
         self.session.commit()
         return ImportResult(status="refreshed", image_id=existing.id)
 
+    def _apply_post_fields(self, post: Post, sd) -> None:
+        """Write a parsed sidecar's post-level fields onto a Post — the SINGLE
+        predicate shared by BOTH ingest paths: the per-media path (_apply_sidecar)
+        and the post-record path (upsert_post_record). Fill-with-non-empty:
+        parse_sidecar yields None for empty values, so a None field is left
+        untouched (an empty feed body never wipes a populated one). raw_metadata +
+        external-link sync always run (latest snapshot). Keeping ONE copy stops
+        the two paths from diverging on how a post body/links get stored — they
+        were verbatim duplicates (#842 DRY pass; see [[feedback_preview_apply_parity]]).
+        """
+        if sd.post_url is not None:
+            post.post_url = sd.post_url
+        if sd.post_title is not None:
+            post.post_title = sd.post_title
+        if sd.post_date is not None:
+            post.post_date = sd.post_date
+        if sd.description is not None:
+            post.description = sd.description
+        if sd.attachment_count is not None:
+            post.attachment_count = sd.attachment_count
+        post.raw_metadata = sd.raw
+        self._sync_external_links(post)
+
     def upsert_post_record(
         self, sidecar: Path, *, artist: Artist | None = None,
         source: Source | None = None,
@@ -785,18 +808,7 @@ class Importer:
         )
         if post.artist_id is None:
             post.artist_id = artist.id
-        if sd.post_url is not None:
-            post.post_url = sd.post_url
-        if sd.post_title is not None:
-            post.post_title = sd.post_title
-        if sd.post_date is not None:
-            post.post_date = sd.post_date
-        if sd.description is not None:
-            post.description = sd.description
-        if sd.attachment_count is not None:
-            post.attachment_count = sd.attachment_count
-        post.raw_metadata = sd.raw
-        self._sync_external_links(post)
+        self._apply_post_fields(post, sd)
         self.session.commit()
         return True
 
@@ -1098,18 +1110,7 @@ class Importer:
             external_post_id=epid,
             artist_id=artist.id,
         )
-        if sd.post_url is not None:
-            post.post_url = sd.post_url
-        if sd.post_title is not None:
-            post.post_title = sd.post_title
-        if sd.post_date is not None:
-            post.post_date = sd.post_date
-        if sd.description is not None:
-            post.description = sd.description
-        if sd.attachment_count is not None:
-            post.attachment_count = sd.attachment_count
-        post.raw_metadata = sd.raw
-        self._sync_external_links(post)
+        self._apply_post_fields(post, sd)
 
         # Race-safe (image_record_id, post_id) upsert — mirrors the
         # _find_or_create_source/post savepoint pattern. The plain