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/gallery_dl.py

@@ -147,6 +147,12 @@ class DownloadResult:
     files_quarantined: int = 0
     quarantined_paths: list[str] = field(default_factory=list)
     written_paths: list[str] = field(default_factory=list)
+    # Native ingester only: post-only sidecar paths for posts that have NO
+    # downloadable media (pure-text posts), so the importer can still upsert the
+    # Post + its body. Empty on the gallery-dl path. Phase 3 imports these via
+    # Importer.upsert_post_record (keyed on external_post_id → updates, never
+    # doubles, the same Post a media import would create).
+    post_record_paths: list[str] = field(default_factory=list)
     stdout: str = ""
     stderr: str = ""
     return_code: int = 0