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/download_service.py

@@ -380,6 +380,22 @@ class DownloadService:
             else:
                 import_summary["errors"] += 1
 
+        # Post-only records: media-less posts (pure-text) the native ingester
+        # captured so the artist archive is complete. Upsert each (keyed on
+        # external_post_id → updates the same Post a media import would create,
+        # never doubles). No file to clean up; the sidecar stays on disk.
+        for rec_str in dl_result.post_record_paths or []:
+            rec_path = Path(rec_str)
+            if not rec_path.exists():  # noqa: ASYNC240
+                continue
+
+            def _upsert(p=rec_path):
+                return self.importer.upsert_post_record(
+                    p, artist=artist, source=source_row,
+                )
+
+            await loop.run_in_executor(None, _upsert)
+
         ev = (await self.async_session.execute(
             select(DownloadEvent).where(DownloadEvent.id == event_id)
         )).scalar_one()