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>

backend/app/services/patreon_downloader.py

@@ -499,6 +499,12 @@ class PatreonDownloader:
         registers no derive_post_url, so `url` is trusted as the permalink — we
         pass the post's attributes.url.
         """
+        return self._write_sidecar_data(post, media_path.with_suffix(".json"))
+
+    def _write_sidecar_data(self, post: dict, sidecar_path: Path) -> Path:
+        """Serialize the post's metadata to `sidecar_path`. Shared by the
+        per-media sidecar (next to each file) and the post-only sidecar
+        (`write_post_record`, for media-less posts)."""
         attrs = post.get("attributes") or {}
         title = attrs.get("title")
         content = attrs.get("content")
@@ -524,6 +530,18 @@ class PatreonDownloader:
             "published_at": published if isinstance(published, str) else None,
             "url": url if isinstance(url, str) else None,
         }
-        sidecar_path = media_path.with_suffix(".json")
         sidecar_path.write_text(json.dumps(data, indent=2))
         return sidecar_path
+
+    def write_post_record(self, post: dict, artist_slug: str) -> Path | None:
+        """Write a post-ONLY sidecar (no media file) for a media-less post, so
+        the importer can still upsert the Post + its body — text posts often hold
+        the only copy of an external <a href> link. Named `_post.json`: the
+        leading underscore keeps it from colliding with a media sidecar
+        (`<NN>_<stem>.json`) and from being resolved as some media file's sidecar
+        by find_sidecar. Returns the path, or None when the post has no id."""
+        if not str(post.get("id") or ""):
+            return None
+        post_dir = self.images_root / artist_slug / "patreon" / _post_dir_name(post)
+        post_dir.mkdir(parents=True, exist_ok=True)
+        return self._write_sidecar_data(post, post_dir / "_post.json")