feat(patreon): capture full post body via adaptive detail-fetch

The feed endpoint (/api/posts) returns `content` empty for many posts, so post
bodies — their formatting, inline <img>, and external <a href> links — were
never captured (the post showed "(no description)"). Enrich an empty feed body
from the per-post detail endpoint (/api/posts/{id}) before writing the importer
sidecar, memoized by mutating the shared post dict so a multi-image post fetches
detail exactly once and fully-seen posts (no fresh download) pay nothing.
Best-effort by design: a body we can't fetch returns None and never fails the
walk. No-doubling and no-clobber-of-populated-body already hold (post upsert is
keyed on external_post_id; an empty body parses to None and isn't applied).

First slice of milestone #64 (rich post capture + faithful rendering +
external-host downloads). Refs FC #830.

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

tests/test_patreon_downloader.py

@@ -7,6 +7,7 @@ via monkeypatching `_run_ytdlp`. PURE module → no DB, so unmarked (fast lane).
 
 from __future__ import annotations
 
+import json
 from pathlib import Path
 
 import pytest
@@ -536,3 +537,43 @@ def test_range_ignored_by_server_refetches_clean(tmp_path, monkeypatch):
     outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
     assert outcomes[0].status == "downloaded"
     assert outcomes[0].path.read_bytes() == full  # clean, not full[:400] + full
+
+
+# -- 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."""
+    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,
+    )
+    post = _post()
+    post["attributes"]["content"] = ""  # feed returned an empty body
+    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
+    assert calls == []