refactor(ingest): post-first — post-record is the sole body writer on the native path (#856)

Milestone #67 step 2. On the native core ingester the Post becomes the single
authoritative record for body/links/metadata, captured once per post by the
post-record; the per-media import only links image provenance + localization.

Before: every per-media sidecar carried the full post body, so a post with N
images wrote the body N+1 times (post-record + N media) — redundant on disk and
a divergence risk (#753). gallery-dl is unchanged (its sidecar is still the only
body source).

- patreon_downloader: the per-media sidecar is now minimal — {category, id,
  source_url} only, no body. `_write_sidecar_data(minimal=True)` skips the body
  resolution + detail-fetch (the post-record, written first in the walk, already
  did it). Body no longer duplicated next to each image.
- importer: new per-instance `post_first` flag (Importer is per-task). When set,
  `_apply_sidecar` still writes source_filehash + provenance + primary_post_id
  but SKIPS `_apply_post_fields` (the post-record owns body/links/raw_metadata,
  so applying a body-less sidecar would clobber raw_metadata + re-sync links off
  empty data). Default False keeps gallery-dl writing post fields.
- download_service: `_phase3_persist` sets importer.post_first =
  uses_native_ingester(platform) — the future-proof seam, so a platform migrating
  onto the native core flips to post-first automatically (step 3). Media imports
  before post-records but both unify on external_post_id, so the post ends with
  its body either way.

Tests: per-media sidecar is minimal + never hits the detail fetcher; attach
post_first=True links provenance/localization but writes no post body/title;
post_first=False (gallery-dl) still applies them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_patreon_downloader.py

@@ -594,40 +594,34 @@ def test_range_ignored_by_server_refetches_clean(tmp_path, monkeypatch):
 # -- content enrichment (adaptive detail-fetch of an empty feed body) -------
 
 
-def test_enriches_empty_content_via_fetcher(tmp_path):
-    """An empty feed body is backfilled from the detail-fetch seam — once per
-    post (memoized on the shared dict) — and lands in the importer sidecar."""
+def test_media_sidecar_is_minimal_post_first(tmp_path):
+    """Post-first (#856): the per-media sidecar carries ONLY image identity
+    (category/id/source_url), NOT the post body — the post-record (`_post.json`)
+    owns the body and is written first in the walk. So download_post never hits
+    the detail fetcher (that's write_post_record's job), and the body is no longer
+    duplicated next to every image."""
     calls: list[str] = []
-
-    def _fetcher(post_id: str) -> str:
-        calls.append(post_id)
-        return "<p>full body</p>"
-
     dl = PatreonDownloader(
         images_root=tmp_path, cookies_path=None, validate=False,
-        session=_FakeSession(), content_fetcher=_fetcher,
+        session=_FakeSession(),
+        content_fetcher=lambda pid: calls.append(pid) or "<p>full body</p>",
     )
     post = _post()
-    post["attributes"]["content"] = ""  # feed returned an empty body
+    post["attributes"]["content"] = ""  # even an empty feed body isn't fetched here
     dl.download_post(post, [_img("a.png"), _img("b.png")], "artist-x")
 
     post_dir = tmp_path / "artist-x" / "patreon" / pd_mod._post_dir_name(post)
     sc = find_sidecar(post_dir / "01_a.png")
     assert sc is not None
-    assert json.loads(sc.read_text())["content"] == "<p>full body</p>"
-    # Fetched ONCE for the post, not once per media item.
-    assert calls == ["1001"]
-
-
-def test_no_enrichment_when_feed_body_present(tmp_path):
-    """A non-empty feed body is trusted as-is; the detail endpoint isn't hit."""
-    calls: list[str] = []
-    dl = PatreonDownloader(
-        images_root=tmp_path, cookies_path=None, validate=False,
-        session=_FakeSession(),
-        content_fetcher=lambda pid: calls.append(pid) or "x",
-    )
-    dl.download_post(_post(), [_img("a.png")], "artist-x")  # _post body is non-empty
+    data = json.loads(sc.read_text())
+    assert data == {
+        "category": "patreon",
+        "id": "1001",
+        "source_url": "https://cdn.patreon.com/a.png",
+    }
+    # No post body/title next to the image; the detail endpoint is never hit here.
+    assert "content" not in data
+    assert "title" not in data
     assert calls == []