refactor(ingest): post-first — post-record is the sole body writer on the native path (#856)

Milestone #67 step 2. On the native core ingester the Post becomes the single
authoritative record for body/links/metadata, captured once per post by the
post-record; the per-media import only links image provenance + localization.

Before: every per-media sidecar carried the full post body, so a post with N
images wrote the body N+1 times (post-record + N media) — redundant on disk and
a divergence risk (#753). gallery-dl is unchanged (its sidecar is still the only
body source).

- patreon_downloader: the per-media sidecar is now minimal — {category, id,
  source_url} only, no body. `_write_sidecar_data(minimal=True)` skips the body
  resolution + detail-fetch (the post-record, written first in the walk, already
  did it). Body no longer duplicated next to each image.
- importer: new per-instance `post_first` flag (Importer is per-task). When set,
  `_apply_sidecar` still writes source_filehash + provenance + primary_post_id
  but SKIPS `_apply_post_fields` (the post-record owns body/links/raw_metadata,
  so applying a body-less sidecar would clobber raw_metadata + re-sync links off
  empty data). Default False keeps gallery-dl writing post fields.
- download_service: `_phase3_persist` sets importer.post_first =
  uses_native_ingester(platform) — the future-proof seam, so a platform migrating
  onto the native core flips to post-first automatically (step 3). Media imports
  before post-records but both unify on external_post_id, so the post ends with
  its body either way.

Tests: per-media sidecar is minimal + never hits the detail fetcher; attach
post_first=True links provenance/localization but writes no post body/title;
post_first=False (gallery-dl) still applies them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/download_service.py

@@ -274,6 +274,14 @@ class DownloadService:
             artist = self.sync_session.get(Artist, ctx["artist_id"])
         source_row = self.sync_session.get(Source, ctx["source_id"])
 
+        # Post-first (#856): on the native ingester the post-record is the sole
+        # writer of the post body/links, so the per-media import must NOT apply post
+        # fields. gallery-dl platforms keep writing them via the sidecar. The
+        # Importer is per-task, so this per-instance flag is safe. uses_native_ingester
+        # is the future-proof seam — a platform migrating onto the native core
+        # flips to post-first automatically (milestone #67, step 3).
+        self.importer.post_first = uses_native_ingester(ctx["platform"])
+
         import_summary = {"attached": 0, "skipped": 0, "errors": 0}
         # Archives detected but captured WITHOUT extracting any image (probe
         # rejected / corrupt / missing extractor backend). Surfaced on the event