fix(posts): link duplicate items to every post + prune bare shells

The native Patreon backfill flooded the feed with bare 'Post <id>' shells
(1589 for Anduo). Root cause: PostAttachment.sha256 was GLOBALLY unique, so a
non-art file reused across posts only ever linked to the first one, and
_capture_attachment created the Post before that dedup check — leaving later
posts with no image and no attachment. Duplicate IMAGES had the mirror gap:
attach_in_place returned duplicate_hash/duplicate_phash before _apply_sidecar,
so the second post got no provenance row, and the feed only rendered via
primary_post_id (one post per image).

Operator requirement: a duplicate item must show on EVERY post it appears in.
Unify the fix as link-not-suppress:

- importer: on duplicate_hash / duplicate_phash(larger_exists), append an
  image_provenance row for the new post (keep primary on the first). Both the
  download path (attach_in_place) and the filesystem path (_import_media).
- post_feed_service: render thumbnails by image_provenance UNION primary_post_id,
  so a cross-posted image shows on every post (and legacy primary-only images
  still show).
- PostAttachment: per-post uniqueness — drop UNIQUE(sha256), add partial
  UNIQUE(post_id, sha256) + partial UNIQUE(sha256) WHERE post_id IS NULL
  (migration 0043); _capture_attachment dedups per-(post,sha) over the shared
  sha-addressed blob, so no post is left bare.
- cleanup: new prune-bare-posts maintenance action (cleanup_service
  _bare_post_conditions shared by preview/count/delete per preview/apply parity;
  admin endpoint; PostMaintenanceCard). Deletes posts with zero image links
  (primary or provenance) AND zero attachments. Run after the feed fix so a
  hidden provenance link spares the post instead of deleting it.

Tests: dup image shows on both posts; dup attachment shows on both posts; feed
renders provenance-linked duplicates; prune-bare delete-path == preview.

Operator redeploys (migration 0043) then runs the prune to clear the shells.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/admin.py

@@ -6,6 +6,7 @@ Five action surfaces:
   DELETE /api/admin/tags/<int:tag_id>              (Tier B)
   POST   /api/admin/tags/<int:dest_id>/merge       (Tier B)
   POST   /api/admin/tags/prune-unused              (Tier A)
+  POST   /api/admin/posts/prune-bare               (Tier A)
   POST   /api/admin/tags/purge-legacy              (Tier A)
   GET    /api/admin/tags/<int:tag_id>/usage-count  (helper)
 
@@ -204,6 +205,27 @@ async def tags_prune_unused():
     return jsonify(result)
 
 
+@admin_bp.route("/posts/prune-bare", methods=["POST"])
+async def posts_prune_bare():
+    """Tier-A: delete bare posts — Post rows with no linked images (primary OR
+    provenance) and no attachments. Dry-run preview list IS the prompt: UI calls
+    with dry_run=true first, shows the count + sample, operator confirms by
+    re-calling with dry_run=false. Same preview/apply-parity predicate as the
+    prune itself, so the preview can't diverge from the delete."""
+    from ..services.cleanup_service import prune_bare_posts
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", False))
+
+    async with get_session() as session:
+        result = await session.run_sync(
+            lambda sync_sess: prune_bare_posts(
+                sync_sess, dry_run=dry_run,
+            )
+        )
+    return jsonify(result)
+
+
 @admin_bp.route("/tags/purge-legacy", methods=["POST"])
 async def tags_purge_legacy():
     """Tier-A: delete legacy IR-migration tags — archive/post/artist