feat(import): surface WHY an archive was captured without extracting images

The recurring "post shows a zip but no images" report had no diagnostic: when
_import_archive captured an archive as a bare PostAttachment — because the
bomb-guard probe rejected it, or extraction yielded zero members (corrupt /
unsupported / missing extractor backend), or it held only non-media files — it
returned status="attached" silently.

Now those paths set ImportResult.error with the specific reason and log a
warning, and download_service records each as {file, reason} under the event's
metadata.unextracted_archives (None when every archive extracted cleanly). So
the next run names exactly which archives failed and why, instead of leaving the
operator to guess. No behaviour change to the happy path.

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

backend/app/services/importer.py

@@ -430,13 +430,17 @@ class Importer:
             self._capture_attachment(
                 source, post=post, artist=artist_use, resolved=True,
             )
-            return ImportResult(status="attached")
+            reason = f"archive probe rejected, captured unextracted: {probe.reason}"
+            log.warning("%s: %s", source.name, reason)
+            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)
         member_ids: list[int] = []
+        member_total = 0
         with extract_archive(source) as members:
             for _name, member_path in members:
+                member_total += 1
                 if not is_supported(member_path):
                     continue  # non-media preserved via the stored archive
                 res = self._import_media(
@@ -453,7 +457,19 @@ class Importer:
                 status="imported", image_id=member_ids[0],
                 member_image_ids=member_ids,
             )
-        return ImportResult(status="attached")
+        # No images landed — surface WHY so a post showing "no images" beside an
+        # archive is diagnosable instead of silent. Zero members usually means
+        # the extractor backend is missing/failed (unar for rar, py7zr for 7z)
+        # or the file is corrupt; non-zero-but-no-images means it held only
+        # non-media files.
+        reason = (
+            "archive extracted but held no supported image/video members"
+            if member_total
+            else "archive yielded no members (unsupported/corrupt, or the "
+            "extractor backend failed)"
+        )
+        log.warning("%s: %s", source.name, reason)
+        return ImportResult(status="attached", error=reason)
 
     def _import_media(
         self, source: Path, attribution_path: Path,