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/importer.py

@@ -710,6 +710,66 @@ class Importer:
         self.session.commit()
         return ImportResult(status="refreshed", image_id=existing.id)
 
+    def upsert_post_record(
+        self, sidecar: Path, *, artist: Artist | None = None,
+        source: Source | None = None,
+    ) -> bool:
+        """Upsert the Post for a post-ONLY sidecar (a media-less post), so the
+        artist archive includes text posts (their body + external links).
+
+        Reuses `_find_or_create_post` (keyed on external_post_id) so it UPDATES
+        the SAME Post a media import would create — never doubles. Fields are
+        FILLED, never clobbered with empty: parse_sidecar yields None for empty
+        values, and a None field is left untouched (an empty feed body never
+        wipes a populated one). Returns True if a Post was upserted, False if the
+        sidecar was unusable (parse failure / no artist)."""
+        try:
+            data = json.loads(sidecar.read_text("utf-8"))
+            if not isinstance(data, dict):
+                raise ValueError("sidecar JSON is not an object")
+        except Exception as exc:
+            log.warning("post-record sidecar parse failed for %s: %s", sidecar, exc)
+            return False
+
+        sd = parse_sidecar(data)
+
+        if artist is None:
+            name = self._sidecar_artist_name(data)
+            artist = self._upsert_artist(name) if name else None
+        if artist is None:
+            log.warning("post-record sidecar %s has no artist; skipping", sidecar)
+            return False
+
+        if source is not None:
+            src = source
+        else:
+            platform = sd.platform or "unknown"
+            src = self._lookup_source_for_sidecar(
+                artist_id=artist.id, platform=platform,
+            )
+
+        epid = sd.external_post_id or sidecar.stem
+        post = self._find_or_create_post(
+            source_id=src.id if src else None,
+            external_post_id=epid,
+            artist_id=artist.id,
+        )
+        if post.artist_id is None:
+            post.artist_id = artist.id
+        if sd.post_url is not None:
+            post.post_url = sd.post_url
+        if sd.post_title is not None:
+            post.post_title = sd.post_title
+        if sd.post_date is not None:
+            post.post_date = sd.post_date
+        if sd.description is not None:
+            post.description = sd.description
+        if sd.attachment_count is not None:
+            post.attachment_count = sd.attachment_count
+        post.raw_metadata = sd.raw
+        self.session.commit()
+        return True
+
     def attach_in_place(
         self,
         path: Path,