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_prosemirror.py

@@ -0,0 +1,108 @@
+"""Unit tests for the Patreon ProseMirror→HTML body converter (#842).
+
+Patreon moved the post body from the flat `content` HTML field (now null) to
+`content_json_string` (a ProseMirror doc). PURE module → no DB, fast lane.
+"""
+
+from __future__ import annotations
+
+import json
+
+from backend.app.utils.prosemirror import post_body_html, prosemirror_to_html
+
+
+def _doc(*nodes) -> str:
+    return json.dumps({"type": "doc", "content": list(nodes)})
+
+
+def test_paragraph_text_and_hardbreak():
+    doc = _doc({
+        "type": "paragraph",
+        "content": [
+            {"type": "text", "text": "Hi there"},
+            {"type": "hardBreak"},
+            {"type": "text", "text": "line two"},
+        ],
+    })
+    assert prosemirror_to_html(doc) == "<p>Hi there<br>line two</p>"
+
+
+def test_marks_bold_italic_link():
+    doc = _doc({
+        "type": "paragraph",
+        "content": [
+            {"type": "text", "marks": [{"type": "bold"}], "text": "B"},
+            {"type": "text", "marks": [{"type": "italic"}], "text": "I"},
+            {
+                "type": "text",
+                "marks": [{"type": "link", "attrs": {"href": "https://x.test/a"}}],
+                "text": "L",
+            },
+        ],
+    })
+    assert prosemirror_to_html(doc) == (
+        '<p><strong>B</strong><em>I</em><a href="https://x.test/a">L</a></p>'
+    )
+
+
+def test_image_node_emits_img_with_cdn_src():
+    doc = _doc({
+        "type": "image",
+        "attrs": {"src": "https://cdn.test/p/post/1/abc/1.gif?token-hash=z", "alt": None},
+    })
+    out = prosemirror_to_html(doc)
+    assert out == '<img src="https://cdn.test/p/post/1/abc/1.gif?token-hash=z" alt="">'
+
+
+def test_lists_and_heading():
+    doc = _doc(
+        {"type": "heading", "attrs": {"level": 2},
+         "content": [{"type": "text", "text": "Status"}]},
+        {"type": "bulletList", "content": [
+            {"type": "listItem", "content": [
+                {"type": "paragraph", "content": [{"type": "text", "text": "one"}]}]},
+        ]},
+    )
+    assert prosemirror_to_html(doc) == (
+        "<h2>Status</h2><ul><li><p>one</p></li></ul>"
+    )
+
+
+def test_text_is_html_escaped():
+    doc = _doc({"type": "paragraph",
+                "content": [{"type": "text", "text": "a < b & c > d"}]})
+    assert prosemirror_to_html(doc) == "<p>a &lt; b &amp; c &gt; d</p>"
+
+
+def test_unknown_node_renders_children_not_dropped():
+    doc = _doc({"type": "weirdBlock", "content": [
+        {"type": "paragraph", "content": [{"type": "text", "text": "keep me"}]}]})
+    assert prosemirror_to_html(doc) == "<p>keep me</p>"
+
+
+def test_empty_and_unparseable_return_none():
+    assert prosemirror_to_html(None) is None
+    assert prosemirror_to_html("") is None
+    assert prosemirror_to_html("   ") is None
+    assert prosemirror_to_html("{not json") is None
+    assert prosemirror_to_html(json.dumps({"type": "doc", "content": []})) is None
+
+
+def test_post_body_html_prefers_legacy_content():
+    attrs = {"content": "<p>legacy</p>",
+             "content_json_string": _doc({"type": "paragraph",
+                                          "content": [{"type": "text", "text": "new"}]})}
+    assert post_body_html(attrs) == "<p>legacy</p>"
+
+
+def test_post_body_html_falls_back_to_json_string():
+    attrs = {"content": None,
+             "content_json_string": _doc({"type": "paragraph",
+                                          "content": [{"type": "text", "text": "new"}]})}
+    assert post_body_html(attrs) == "<p>new</p>"
+
+
+def test_post_body_html_none_when_both_empty():
+    assert post_body_html({"content": None, "content_json_string": None}) is None
+    assert post_body_html({}) is None
+    assert post_body_html(None) is None