docs(ingest): document the post-first migration contract at the native seam (#857)

Milestone #67 step 3. Spell out, at the IngestCore.run seam resolution, that
post_record_key + write_post_record are the post-first contract a platform
implements when migrating onto the native core ingester — the post-record owns
the body/links, the per-media sidecar carries image identity only. The import
side is already self-enforcing via uses_native_ingester → importer.post_first.
Durable directive recorded as FC project rule #120.

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

backend/app/services/ingest_core.py

@@ -141,9 +141,18 @@ class Ingester:
         # tick has no resumable backfill state.
         checkpoint = mode in ("backfill", "recovery", "recapture")
         ledger_key = self._ledger_key
-        # Optional seams (Patreon native ingester): capture pure-text posts that
-        # have NO downloadable media so the artist archive is complete. Absent on
-        # stub clients/downloaders (unit tests) → media-less posts skipped as before.
+        # POST-FIRST CONTRACT (milestone #67): these two optional seams make a
+        # platform "post-first" on the native core ingester — the post-record is
+        # the single authoritative writer of the post body/links/metadata, and the
+        # per-media sidecar carries image identity only (download_service flips
+        # importer.post_first via uses_native_ingester, so the import side follows
+        # automatically). A platform migrating off gallery-dl onto the native core
+        # adopts post-first by implementing BOTH:
+        #   client.post_record_key(post)   -> (ledger_key, post_id) | None  (gate)
+        #   downloader.write_post_record(post, artist_slug) -> PostRecordOutcome
+        # Absent on stub clients/downloaders (unit tests) and on not-yet-migrated
+        # platforms → media-less posts are skipped as before and the body still
+        # comes from the per-media sidecar (gallery-dl path). See [[post-first-ingest-contract]].
         post_record_key = getattr(self.client, "post_record_key", None)
         write_post_record = getattr(self.downloader, "write_post_record", None)
         start = time.monotonic()