fix(patreon): read post body from content_json_string (ProseMirror), not the dead content field (#842)

THE empty-body root cause. Patreon deprecated the flat `content` HTML field —
it returns null on the feed AND the detail endpoint, for every post type
(confirmed against the live API: all 135 StickySpoodge posts, text_only/
image_file/poll alike). The real body now lives in `content_json_string` (a
ProseMirror/TipTap doc), returned only under the DEFAULT post fieldset — a sparse
fields[post]=content request omits it. Not credential, not post_type: a request
shape gone stale.

- NEW utils/prosemirror.py: ProseMirror doc -> HTML (paragraphs, marks
  bold/italic/underline/strike/code/link, hardBreak, inline images, lists,
  headings; unknown nodes degrade to children). post_body_html(attrs) = the one
  resolver: legacy content HTML else convert content_json_string.
- patreon_client: add content_json_string to the feed _FIELDS_POST; rewrite
  fetch_post_detail_content to use the DEFAULT fieldset (no sparse fields[post])
  and resolve via post_body_html (replaces the wrong sparse req + full-fetch
  fallback).
- patreon_downloader._write_sidecar_data: resolve body via post_body_html
  (feed content_json_string) before the detail-fetch; memoize resolved HTML.
- tests: prosemirror converter unit tests; client legacy + content_json_string
  paths; contract pins content_json_string.

Inline <img> nodes carry the CDN filehash → bodies now feed Phase-2 localization.

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

tests/test_patreon_client.py

@@ -358,7 +358,10 @@ def test_request_sleep_paces_before_fetch(monkeypatch):
 # -- fetch_post_detail_content (best-effort full-body enrichment) ----------
 
 
-def test_fetch_post_detail_content_returns_body(monkeypatch):
+def test_fetch_post_detail_content_returns_legacy_content(monkeypatch):
+    """Old posts still carry the flat `content` HTML — returned as-is. The detail
+    fetch uses the DEFAULT fieldset (no sparse `fields[post]`, which nulls the
+    body) — #842."""
     payload = {"data": {"id": "42", "attributes": {"content": "<p>full body</p>"}}}
     captured: dict = {}
     client = PatreonClient(cookies_path=None)
@@ -371,37 +374,28 @@ def test_fetch_post_detail_content_returns_body(monkeypatch):
     monkeypatch.setattr(client._session, "get", _get)
     assert client.fetch_post_detail_content("42") == "<p>full body</p>"
     assert captured["url"].endswith("/api/posts/42")
-    assert captured["params"]["fields[post]"] == "content"
+    # No sparse fieldset — that's what nulled the body on the live API.
+    assert not captured["params"] or "fields[post]" not in captured["params"]
+
+
+def test_fetch_post_detail_content_converts_content_json_string(monkeypatch):
+    """Current posts: `content` is null and the body lives in the
+    `content_json_string` ProseMirror doc → converted to HTML (#842)."""
+    doc = (
+        '{"type":"doc","content":[{"type":"paragraph","content":'
+        '[{"type":"text","text":"Hello"}]}]}'
+    )
+    payload = {"data": {"attributes": {"content": None, "content_json_string": doc}}}
+    client = _client_returning(monkeypatch, _FakeResp(200, json_data=payload))
+    assert client.fetch_post_detail_content("42") == "<p>Hello</p>"
 
 
 def test_fetch_post_detail_content_empty_body_is_none(monkeypatch):
-    payload = {"data": {"attributes": {"content": "   "}}}
+    payload = {"data": {"attributes": {"content": "   ", "content_json_string": ""}}}
     client = _client_returning(monkeypatch, _FakeResp(200, json_data=payload))
     assert client.fetch_post_detail_content("42") is None
 
 
-def test_fetch_post_detail_content_falls_back_to_full_fetch(monkeypatch):
-    """#842: when the sparse fields[post]=content request returns null (Patreon
-    does this for polls / embeds / body-only posts), a one-time FULL re-fetch
-    recovers the body — so inline-only / no-media posts still get their text."""
-    client = PatreonClient(cookies_path=None)
-    seen_params: list = []
-
-    def _get(url, params=None, timeout=None):
-        seen_params.append(params or {})
-        if "fields[post]" in (params or {}):
-            return _FakeResp(200, json_data={"data": {"attributes": {"content": None}}})
-        return _FakeResp(200, json_data={
-            "data": {"attributes": {"content": "<p>body via full</p>", "post_type": "poll"}}
-        })
-
-    monkeypatch.setattr(client._session, "get", _get)
-    assert client.fetch_post_detail_content("42") == "<p>body via full</p>"
-    # Sparse request first, then a full re-fetch without the sparse fieldset.
-    assert "fields[post]" in seen_params[0]
-    assert "fields[post]" not in seen_params[1]
-
-
 def test_fetch_post_detail_content_blank_id_skips_request(monkeypatch):
     calls: list = []
     client = PatreonClient(cookies_path=None)