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_client.py

@@ -406,3 +406,16 @@ def test_fetch_post_detail_content_swallows_http_and_transport_errors(monkeypatc
 
     monkeypatch.setattr(client._session, "get", _boom)
     assert client.fetch_post_detail_content("42") is None
+
+
+# -- post_record_key (media-less post seen-ledger gate) --------------------
+
+
+def test_post_record_key_returns_synthetic_key_and_id():
+    assert PatreonClient.post_record_key({"id": "987"}) == ("post:987", "987")
+    assert PatreonClient.post_record_key({"id": 987}) == ("post:987", "987")
+
+
+def test_post_record_key_none_without_id():
+    assert PatreonClient.post_record_key({}) is None
+    assert PatreonClient.post_record_key({"id": None}) is None