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/download_service.py

@@ -269,6 +269,11 @@ class DownloadService:
         source_row = self.sync_session.get(Source, ctx["source_id"])
 
         import_summary = {"attached": 0, "skipped": 0, "errors": 0}
+        # Archives detected but captured WITHOUT extracting any image (probe
+        # rejected / corrupt / missing extractor backend). Surfaced on the event
+        # so a post showing "no images" beside a zip is diagnosable (plan
+        # follow-up 2026-06-06 — the recurring archive-association report).
+        unextracted_archives: list[dict] = []
         bytes_downloaded = 0
 
         loop = asyncio.get_running_loop()
@@ -322,6 +327,17 @@ class DownloadService:
                 # mirror duplicate_hash cleanup so we don't keep two copies.
                 # Operator-flagged 2026-06-02 (Lustria OST zip).
                 import_summary["attached"] += 1
+                # An archive captured WITHOUT extracting any image carries a
+                # reason on result.error — record it so the event explains the
+                # "no images beside a zip" symptom instead of staying silent.
+                if result.error:
+                    unextracted_archives.append(
+                        {"file": path.name, "reason": result.error}
+                    )
+                    log.warning(
+                        "archive captured unextracted (%s): %s",
+                        path.name, result.error,
+                    )
                 try:
                     bytes_downloaded += path.stat().st_size  # noqa: ASYNC240
                 except OSError:
@@ -403,6 +419,10 @@ class DownloadService:
             "duration_seconds": dl_result.duration_seconds,
             "quarantined_paths": dl_result.quarantined_paths or None,
             "import_summary": import_summary,
+            # Archives detected but captured without extracting an image — the
+            # recurring "post shows a zip but no images" report. Each entry is
+            # {file, reason}; None when every archive extracted cleanly.
+            "unextracted_archives": unextracted_archives or None,
         }
         await self._update_source_health(
             source_id=ctx["source_id"], status=status, error_message=ev.error,