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  provenance row for the post (new + superseded + deduped members), resolving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>

frontend/src/components/modal/ProvenancePanel.vue

@@ -70,7 +70,9 @@
     </div>
 
     <div v-if="attachments.length" class="fc-prov__attach">
-      <h4 class="fc-prov__attach-title">Attachments ({{ attachments.length }})</h4>
+      <h4 class="fc-prov__attach-title">
+        {{ attachments.length === 1 ? 'Attachment' : `Attachments (${attachments.length})` }}
+      </h4>
       <!-- Scroll-capped: a single post can carry dozens of archives (HR
            bundle posts), which previously ballooned the panel past the
            viewport. Mirror the cards' independent-scroll treatment. -->