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  provenance row for the post (new + superseded + deduped members), resolving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

alembic/versions/0055_image_provenance_from_attachment.py

@@ -0,0 +1,55 @@
+"""image_provenance: from_attachment_id (which archive an image was extracted from)
+
+Milestone #87. When an image is pulled out of a .zip/.rar, record WHICH archive
+PostAttachment it came from, so the provenance UI can show the single archive a
+file lives inside instead of every attachment on the post. Nullable FK with
+ON DELETE SET NULL — a loose (non-archive) download leaves it NULL, and deleting
+the archive attachment forgets the linkage without destroying the (image, post)
+provenance edge. Existing rows are NULL until the reextract backfill stamps them.
+
+Revision ID: 0055
+Revises: 0054
+Create Date: 2026-06-22
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0055"
+down_revision: Union[str, None] = "0054"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "image_provenance",
+        sa.Column("from_attachment_id", sa.Integer(), nullable=True),
+    )
+    op.create_index(
+        "ix_image_provenance_from_attachment_id",
+        "image_provenance",
+        ["from_attachment_id"],
+    )
+    op.create_foreign_key(
+        "fk_image_provenance_from_attachment",
+        "image_provenance",
+        "post_attachment",
+        ["from_attachment_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "fk_image_provenance_from_attachment",
+        "image_provenance",
+        type_="foreignkey",
+    )
+    op.drop_index(
+        "ix_image_provenance_from_attachment_id",
+        table_name="image_provenance",
+    )
+    op.drop_column("image_provenance", "from_attachment_id")