fix(downloads): enqueue thumbnail + ML tasks per attached image

Operator-flagged 2026-06-01: downloaded images stayed at
thumbnail_path=NULL until a periodic backfill sweep picked them up,
surfacing as broken-thumbnail tiles in the gallery for hours after
the download landed.

Importer.attach_in_place deliberately skips inline thumbnail
generation (importer.py:591-592) so the import queue stays moving —
the CALLING task is responsible for the enqueue. tasks/import_file.py
already did this (line 228-239). tasks/download.py / download_service
did not — every gallery-dl-attached image landed un-thumbnailed.

Fix in download_service._phase3_persist: after each
`attach_in_place` returning status in (imported, superseded), fan out
`generate_thumbnail.delay()` + `tag_and_embed.delay()` for each
image_id. Lazy import avoids circular-import risk between
download_service and the celery task modules that depend on it.

Mirrors the existing pattern verbatim — single source of truth for
"what fires after a successful attach" remains a comment in two
places (filesystem-import task, download orchestrator) rather than
a shared helper, because the contexts differ enough (sync session vs
async orchestrator) that abstracting would obscure more than it'd
share.

Test covers the happy-path with two attached files: both get the
thumbnail enqueue AND the ML enqueue, with image_ids drawn from
ImportResult (so future supersede-on-attach paths stay covered).

backend/app/services/download_service.py

@@ -260,6 +260,25 @@ class DownloadService:
                     bytes_downloaded += path.stat().st_size  # noqa: ASYNC240
                 except OSError:
                     pass
+                # Enqueue thumbnail + ML for newly-attached images, matching
+                # the filesystem-import path (tasks/import_file.py:228-239).
+                # Importer.attach_in_place deliberately skips inline thumb
+                # generation to keep the import queue moving; the calling
+                # task is responsible for the enqueue. Operator-flagged
+                # 2026-06-01: without this, every downloaded image stayed
+                # at thumbnail_path=NULL until a periodic backfill swept
+                # it up, surfacing as broken-thumbnail tiles in the gallery
+                # for hours after a download landed. Lazy import to avoid
+                # circular-import risk between this service and the
+                # tasks/* modules that import it.
+                from ..tasks.ml import tag_and_embed
+                from ..tasks.thumbnail import generate_thumbnail
+                ids = list(result.member_image_ids)
+                if result.image_id is not None and result.image_id not in ids:
+                    ids.append(result.image_id)
+                for img_id in ids:
+                    generate_thumbnail.delay(img_id)
+                    tag_and_embed.delay(img_id)
             elif result.status == "skipped" and result.skip_reason and result.skip_reason.value in (
                 "duplicate_hash", "duplicate_phash",
             ):