feat(external): zip-parity provenance/tagging + thorough worker logging

Operator-requested: a worker download must be tagged + provenance-associated
exactly like an extracted zip, and the path must log well (we won't get it right
first try).

- _route_files now mirrors download_service._phase3_persist branch-for-branch:
  imported/superseded → collect member_image_ids+image_id (provenance-linked via
  the synthesized sidecar, same as extracted-zip members) → caller enqueues
  tag_and_embed + generate_thumbnail; attached → drop on-disk original, and warn
  on an UNEXTRACTED archive (#718 symptom); skipped duplicate → unlink; failed →
  unlink + warn.
- Logging at every stage: start (link/host/post/artist/attempt/url), requeue,
  fetch result (files/bytes) or fetch failure, per-file import decision, dead-
  letter transitions, and done (files/images/duration).
- Parity test: an archive downloaded by the worker is extracted, provenance-
  linked to the SAME post, and tag_and_embed+generate_thumbnail are queued for
  exactly the member images.

Refs FC #830.

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

tests/test_external_worker.py

@@ -1,15 +1,39 @@
 """Integration tests for the external-link download worker (tasks/external)."""
 
+import io
+import zipfile
+
 import pytest
+from PIL import Image
 from sqlalchemy import func, select
 
 import backend.app.tasks.external as ext
-from backend.app.models import Artist, ExternalLink, Post, PostAttachment, Source
+from backend.app.models import (
+    Artist,
+    ExternalLink,
+    ImageProvenance,
+    Post,
+    PostAttachment,
+    Source,
+)
 from backend.app.services.external_fetch import FetchResult
 
 pytestmark = pytest.mark.integration
 
 
+def _structured_jpeg_bytes():
+    # A non-uniform image so it survives the import filters (a solid color would
+    # phash-collapse / fail thresholds).
+    im = Image.new("RGB", (256, 256))
+    px = im.load()
+    for y in range(256):
+        for x in range(256):
+            px[x, y] = (x, y, (x + y) % 256)
+    buf = io.BytesIO()
+    im.save(buf, "JPEG")
+    return buf.getvalue()
+
+
 class _FakeLock:
     def acquire(self, blocking=False):
         return True
@@ -136,3 +160,44 @@ def test_sweep_enqueues_pending_and_retryable(db_sync, monkeypatch):
     out = ext.sweep_external_links()
     assert out["enqueued"] == 2
     assert set(enqueued) == {l1.id, l2.id}
+
+
+def test_downloaded_archive_gets_provenance_and_tagging(db_sync, tmp_path, monkeypatch):
+    """An archive downloaded by the worker is extracted, provenance-linked to the
+    post, AND queued for tagging + thumbnails — exactly like an extracted zip on
+    the normal download path."""
+    post, link = _seed(db_sync, host="mega")
+    link_id = link.id
+
+    monkeypatch.setattr(ext, "IMAGES_ROOT", tmp_path)
+    monkeypatch.setattr(ext, "_redis", lambda: _FakeRedis())
+
+    def fake_fetch(host, url, dest_dir, *, timeout, should_stop=lambda: False):
+        dest_dir.mkdir(parents=True, exist_ok=True)
+        zpath = dest_dir / "pack.zip"
+        with zipfile.ZipFile(zpath, "w") as z:
+            z.writestr("art.jpg", _structured_jpeg_bytes())
+        return FetchResult(files=[zpath], bytes=zpath.stat().st_size)
+
+    monkeypatch.setattr(ext, "fetch_external", fake_fetch)
+
+    tagged, thumbed = [], []
+    import backend.app.tasks.ml as ml_mod
+    import backend.app.tasks.thumbnail as thumb_mod
+    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda i: tagged.append(i))
+    monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda i: thumbed.append(i))
+
+    out = ext.fetch_external_link(link_id)
+    assert out["images"] >= 1
+
+    db_sync.expire_all()
+    assert db_sync.get(ExternalLink, link_id).status == "downloaded"
+    # Extracted member is provenance-linked to the SAME post (as extracted zips).
+    prov = db_sync.execute(
+        select(ImageProvenance).where(ImageProvenance.post_id == post.id)
+    ).scalars().all()
+    assert len(prov) >= 1
+    member_ids = {p.image_record_id for p in prov}
+    # Tagging + thumbnails queued for exactly those member images.
+    assert set(tagged) == member_ids
+    assert set(thumbed) == member_ids