feat(provenance): capture which archive an extracted image came from (#87)

Images pulled out of a .zip/.rar previously kept no record of WHICH archive
they came from — the member->archive link was computed during extraction and
discarded, leaving only image->post. So the provenance modal could only scope
attachments to the whole post, showing every archive a 'High Resolution Files'
bundle carried instead of the one a given file lives in.

- ImageProvenance.from_attachment_id: nullable FK -> post_attachment.id
  (SET NULL), migration 0055.
- importer: _import_archive stamps from_attachment_id on every member's
  provenance row for the post (new + superseded + deduped members), resolving
  the archive's own PostAttachment by (post, sha). Post-pass UPDATE, NULL-only
  and idempotent, so it doesn't touch the dedup/supersede branches and the
  backfill is safe to re-run. Nested members link to the outer stored archive.
- provenance_service.for_image: when the originating post's provenance row
  records from_attachment_id, return ONLY that archive; else fall back to the
  primary-post scoping from 068def2.
- ProvenancePanel: heading pluralizes ('Attachment' for a single file).
- Backfill: re-running reextract_archive_attachments (ArchiveReextractCard)
  routes through _import_archive and stamps existing rows — no new code.

Tests: capture stamps on fresh import, nested-archive attribution, per-post
archive on dedup; for_image filters to the containing archive; reextract
backfill stamps the link.

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

backend/app/services/provenance_service.py

@@ -66,6 +66,10 @@ class ProvenanceService:
         ).scalars().all()
         return [_attachment_dict(a) for a in rows]
 
+    async def _attachment_by_id(self, attachment_id: int) -> list[dict]:
+        att = await self.session.get(PostAttachment, attachment_id)
+        return [_attachment_dict(att)] if att is not None else []
+
     async def for_image(self, image_id: int) -> dict | None:
         rec = await self.session.get(ImageRecord, image_id)
         if rec is None:
@@ -85,22 +89,33 @@ class ProvenanceService:
         )
         rows = (await self.session.execute(stmt)).all()
         post_ids = [ip.post_id for ip, _p, _s, _a in rows]
-        # Scope attachments to the image's ORIGINATING post only, not every
-        # post it's pHash-linked to. An image dupe gets a provenance row per
-        # post it reappears in (enrich-on-duplicate), and one of those is
-        # often a "High Resolution Files" mega-bundle carrying dozens of
-        # unrelated archives — aggregating across all linked posts ballooned
-        # the panel with files that have nothing to do with this image.
-        # primary_post_id is the post this file was actually captured from;
-        # fall back to all linked posts only when it's unset (older rows /
-        # filesystem imports). NB: FC stores archives as opaque blobs and
-        # never records which archive an extracted image came from, so we
-        # cannot scope tighter than the post — see milestone for the
-        # image->archive capture work.
-        attach_post_ids = (
-            [rec.primary_post_id] if rec.primary_post_id is not None else post_ids
+        # Prefer the EXACT archive this file came out of (milestone #87): if the
+        # originating post's provenance row records from_attachment_id, the image
+        # was extracted from that one .zip/.rar, so show only it — not the dozens
+        # of unrelated archives a "High Resolution Files" bundle post carries.
+        from_att_id = next(
+            (
+                ip.from_attachment_id
+                for ip, _p, _s, _a in rows
+                if ip.post_id == rec.primary_post_id
+                and ip.from_attachment_id is not None
+            ),
+            None,
         )
-        attachments = await self._attachments_for_posts(attach_post_ids)
+        if from_att_id is not None:
+            attachments = await self._attachment_by_id(from_att_id)
+        else:
+            # No recorded containing archive (loose download, or pre-backfill):
+            # scope to the originating post only, not every pHash-linked post.
+            # primary_post_id is the post this file was actually captured from;
+            # fall back to all linked posts when it's unset (older rows /
+            # filesystem imports).
+            attach_post_ids = (
+                [rec.primary_post_id]
+                if rec.primary_post_id is not None
+                else post_ids
+            )
+            attachments = await self._attachments_for_posts(attach_post_ids)
         return {
             "image_id": image_id,
             "provenance": [