fix(recapture): link on-disk images to their post (#1288)

Recapture disk-skips already-downloaded media, and upsert_post_record only
writes Post fields — so a pre-existing image (e.g. one pulled under the old
gallery-dl path, imported bare with no post) stays orphaned even after its post
record is (re)written. Confirmed on the operator's instance: 329 pixiv images
with primary_post_id NULL, 694 pixiv posts with content but no linked images, 0
duplicate posts.

Fix: the recapture relink channel now carries the media's post_id (2- → 3-tuple
path/url/post_id), and phase 3 calls importer.link_existing_image_to_post — match
the on-disk image by path, find its Post by (source, external_post_id), upsert
image_provenance + primary_post_id. Factored the provenance-linking out of
_apply_sidecar into a shared _attach_provenance so the fresh-import and
recapture-backlink paths can't diverge. Idempotent; generic across native
platforms (no-op for already-linked Patreon/SubscribeStar). Re-running recapture
now repairs orphaned images; future walks never orphan.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/ingest_core.py

@@ -179,10 +179,11 @@ class Ingester:
         written: list[str] = []
         post_records: list[str] = []
         quarantined_paths: list[str] = []
-        # #830 recapture: (on-disk path, CDN source_url) pairs for already-present
-        # media, so phase 3 can backfill the ImageRecord's source_filehash WITHOUT
-        # re-downloading or unlinking the file. Empty outside recapture mode.
-        relink: list[tuple[str, str]] = []
+        # #830 recapture: (on-disk path, CDN source_url, post_id) triples for
+        # already-present media, so phase 3 can (a) backfill the ImageRecord's
+        # source_filehash and (b) link the on-disk image to its Post (#1288) —
+        # WITHOUT re-downloading or unlinking the file. Empty outside recapture.
+        relink: list[tuple[str, str, str]] = []
         downloaded = 0
         errors = 0
         quarantined = 0
@@ -410,12 +411,15 @@ class Ingester:
                         to_clear.append(key)
                         skipped_count += 1
                         consecutive_seen += 1
-                        # #830 recapture: surface (on-disk path, CDN url) so phase
-                        # 3 can backfill source_filehash for inline-image
-                        # localization — a SEPARATE non-deleting channel, never the
-                        # import list (which would unlink the file, per above).
+                        # #830/#1288 recapture: surface (on-disk path, CDN url,
+                        # post_id) so phase 3 can backfill source_filehash AND link
+                        # the on-disk image to its Post — a SEPARATE non-deleting
+                        # channel, never the import list (which would unlink the
+                        # file, per above).
                         if recapture and outcome.path is not None:
-                            relink.append((str(outcome.path), media_item.url))
+                            relink.append(
+                                (str(outcome.path), media_item.url, media_item.post_id)
+                            )
                     elif outcome.status == "skipped_seen":
                         skipped_count += 1
                         consecutive_seen += 1