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_sidecar_import.py

@@ -8,6 +8,7 @@ from PIL import Image
 from sqlalchemy import func, select
 
 from backend.app.models import (
+    Artist,
     ImageProvenance,
     ImageRecord,
     ImportSettings,
@@ -164,8 +165,6 @@ def test_no_artist_anywhere_skips_provenance(importer, import_layout):
 
 
 def test_sidecar_artist_used_when_no_folder_artist(importer, import_layout):
-    from backend.app.models import Artist
-
     import_root, _ = import_layout
     m = import_root / "e.jpg"                 # root → no folder artist
     _split(m, "v")
@@ -183,3 +182,47 @@ def test_sidecar_artist_used_when_no_folder_artist(importer, import_layout):
     assert importer.session.execute(
         select(func.count()).select_from(Source)
     ).scalar_one() == 0
+
+
+def test_upsert_post_record_creates_media_less_post(importer, import_layout):
+    """A post-only sidecar (no media) upserts a Post WITH its body — text posts
+    are captured even when they have nothing to download."""
+    import_root, _ = import_layout
+    artist = Artist(name="Alice", slug="alice")
+    importer.session.add(artist)
+    importer.session.flush()
+    sc = import_root / "Alice" / "_post.json"
+    sc.parent.mkdir(parents=True, exist_ok=True)
+    sc.write_text(json.dumps({
+        "category": "patreon", "id": 777,
+        "url": "https://patreon.com/posts/777", "title": "Text only",
+        "content": "<p>links below: mega</p>",
+        "published_at": "2023-08-01T00:00:00Z",
+    }))
+    assert importer.upsert_post_record(sc, artist=artist) is True
+    post = importer.session.execute(select(Post)).scalar_one()
+    assert post.external_post_id == "777"
+    assert post.post_title == "Text only"
+    assert post.description == "<p>links below: mega</p>"
+    assert post.post_url == "https://patreon.com/posts/777"
+    assert post.post_date is not None
+
+
+def test_upsert_post_record_idempotent_no_double(importer, import_layout):
+    """Re-running upsert_post_record UPDATES the same Post (keyed on
+    external_post_id) — never doubles."""
+    import_root, _ = import_layout
+    artist = Artist(name="Bob", slug="bob")
+    importer.session.add(artist)
+    importer.session.flush()
+    sc = import_root / "Bob" / "_post.json"
+    sc.parent.mkdir(parents=True, exist_ok=True)
+    sc.write_text(json.dumps({
+        "category": "patreon", "id": 888, "title": "T",
+        "content": "<p>body</p>", "url": "https://patreon.com/posts/888",
+    }))
+    assert importer.upsert_post_record(sc, artist=artist) is True
+    assert importer.upsert_post_record(sc, artist=artist) is True
+    assert importer.session.execute(
+        select(func.count()).select_from(Post)
+    ).scalar_one() == 1