fix: a non-image file lands on the source it was downloaded for, and the undated shells it left are folded into their real posts (#4435)

The attachment path looked the post's source up by (artist, platform),
taking the artist's lowest-id source. A Discord artist has one source per
channel, so every archive or pdf from a later channel became an undated
second post on the first channel's source. _post_for_sidecar now takes
the downloading source, as upsert_post_record and _apply_sidecar already
did. Migration 0114 re-points each shell's attachments to its dated twin
and deletes the shell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/importer.py

@@ -426,11 +426,19 @@ class Importer:
         return self._upsert_artist(name) if name else None
 
     def _post_for_sidecar(
-        self, source: Path, artist: Artist | None
+        self, source: Path, artist: Artist | None,
+        *, source_row: Source | None = None,
     ) -> Post | None:
         """If a sidecar sits next to `source`, ensure its Source+Post
         exist (idempotent) and return the Post — so attachments can link
-        to the same Post the per-member _apply_sidecar will reuse."""
+        to the same Post the per-member _apply_sidecar will reuse.
+
+        `source_row` is the subscription being downloaded, when there is one,
+        and wins over the (artist, platform) lookup — as it does in
+        `upsert_post_record`. The lookup takes the artist's FIRST source on the
+        platform, which is right only while an artist has one: a Discord artist
+        has one per channel, and every non-image file from a later channel was
+        filed under the first as an undated second post (#4435)."""
         sc = find_sidecar(source)
         if sc is None or artist is None:
             return None
@@ -442,10 +450,13 @@ class Importer:
             log.warning("sidecar parse failed for %s: %s", sc, exc)
             return None
         sd = parse_sidecar(data)
-        platform = sd.platform or "unknown"
-        src = self._lookup_source_for_sidecar(
-            artist_id=artist.id, platform=platform,
-        )
+        if source_row is not None:
+            src = source_row
+        else:
+            platform = sd.platform or "unknown"
+            src = self._lookup_source_for_sidecar(
+                artist_id=artist.id, platform=platform,
+            )
         epid = sd.external_post_id or sc.stem
         return self._find_or_create_post(
             source_id=src.id if src else None,
@@ -543,7 +554,7 @@ class Importer:
             # nothing silently vanishes, matching extract_archive's
             # fail-soft contract.
             artist_use = artist if artist is not None else self._resolve_artist(source)
-            post = self._post_for_sidecar(source, artist_use)
+            post = self._post_for_sidecar(source, artist_use, source_row=source_row)
             self._capture_attachment(
                 source, post=post, artist=artist_use, resolved=True,
             )
@@ -552,7 +563,7 @@ class Importer:
             return ImportResult(status="attached", error=reason)
 
         artist_use = artist if artist is not None else self._resolve_artist(source)
-        post = self._post_for_sidecar(source, artist_use)
+        post = self._post_for_sidecar(source, artist_use, source_row=source_row)
         member_ids: list[int] = []
         # Every member image touched (new + superseded + deduped), so the
         # from_attachment_id stamp below covers files that already existed in the
@@ -1218,7 +1229,10 @@ class Importer:
                 path, artist=artist, source_row=source,
             )
         if not is_supported(path):
-            post = self._post_for_sidecar(path, artist) if artist else None
+            post = (
+                self._post_for_sidecar(path, artist, source_row=source)
+                if artist else None
+            )
             return self._capture_attachment(
                 path, post=post, artist=artist, resolved=True,
             )