feat(patreon): capture media-less/text-only posts (post-only records)

Today the ingest core does `if not media: continue`, so a post with no
downloadable media (a pure-text post — which often holds the ONLY copy of an
external mega/gdrive/pixeldrain link) never upserts a Post. Now the native
ingester emits a post-only sidecar (`_post.json`) for every media-less post,
gated through the seen-ledger via a synthetic `post:<id>` key so the body is
detail-fetched + recorded ONCE (not re-walked every tick); recovery bypasses
the gate. Phase 3 imports these via Importer.upsert_post_record, keyed on
external_post_id so it UPDATES the same Post a media import would create —
never doubles, never clobbers a populated body with an empty one.

- gallery_dl.py: DownloadResult.post_record_paths (default []; gallery-dl path
  unaffected — all constructions are keyword).
- ingest_core.py: media-less branch (optional client/downloader seams via
  getattr; stub clients in tests skip it as before).
- patreon_client.py: post_record_key(post). patreon_downloader.py:
  write_post_record + _write_sidecar_data refactor (shared serializer).
- importer.py: upsert_post_record. download_service.py: phase-3 import loop.
- tests: client/downloader/ingester (gate + recovery)/importer (no-double).

Slice 0b of milestone #64. Refs FC #830.

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

tests/test_patreon_downloader.py

@@ -577,3 +577,37 @@ def test_no_enrichment_when_feed_body_present(tmp_path):
     )
     dl.download_post(_post(), [_img("a.png")], "artist-x")  # _post body is non-empty
     assert calls == []
+
+
+# -- write_post_record (media-less / text-only post capture) ----------------
+
+
+def test_write_post_record_writes_enriched_post_only_sidecar(tmp_path):
+    calls: list[str] = []
+
+    def _fetcher(post_id: str) -> str:
+        calls.append(post_id)
+        return "<p>text post body</p>"
+
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False,
+        session=_FakeSession(), content_fetcher=_fetcher,
+    )
+    post = _post()
+    post["attributes"]["content"] = ""  # media-less post, empty feed body
+    sc = dl.write_post_record(post, "artist-x")
+
+    assert sc is not None
+    assert sc.name == "_post.json"  # can't collide with a media `NN_*.json`
+    data = json.loads(sc.read_text())
+    assert data["id"] == "1001"
+    assert data["content"] == "<p>text post body</p>"
+    assert calls == ["1001"]
+
+
+def test_write_post_record_none_without_post_id(tmp_path):
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False,
+        session=_FakeSession(),
+    )
+    assert dl.write_post_record({"id": "", "attributes": {}}, "artist-x") is None