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  provenance row for the post (new + superseded + deduped members), resolving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/models/image_provenance.py

@@ -41,6 +41,16 @@ class ImageProvenance(Base):
     source_id: Mapped[int | None] = mapped_column(
         ForeignKey("source.id", ondelete="SET NULL"), nullable=True, index=True
     )
+    # The archive PostAttachment this image was extracted FROM, when it came
+    # out of a .zip/.rar rather than as a loose file (milestone #87). Lets the
+    # provenance UI show the exact archive a file lives inside instead of every
+    # attachment on the post. NULL for loose downloads and pre-backfill rows.
+    # SET NULL so deleting the archive attachment never destroys the (image,
+    # post) edge — it just forgets which archive it came from.
+    from_attachment_id: Mapped[int | None] = mapped_column(
+        ForeignKey("post_attachment.id", ondelete="SET NULL"),
+        nullable=True, index=True,
+    )
     captured_metadata: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     captured_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()