fix(provenance): scope attachments to originating post + scroll-cap the list

The Attachments section aggregated PostAttachment rows across EVERY post an
image was pHash-linked to. When one of those was a 'High Resolution Files'
mega-bundle (dozens of unrelated archives), the list ballooned past the
viewport and overwhelmed the modal's right rail.

- for_image() now scopes attachments to ImageRecord.primary_post_id (the post
  the file was actually captured from), falling back to all linked posts only
  when primary_post_id is unset (older rows / filesystem imports).
- ProvenancePanel wraps the list in a max-height scroll container with a count
  in the heading, mirroring the cards' independent-scroll treatment.

Note: FC stores archives as opaque blobs and never records which archive an
extracted image came from, so attachments can't yet be scoped tighter than the
post. Capturing image->archive containment is tracked as separate work.

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

backend/app/services/provenance_service.py

@@ -85,7 +85,22 @@ class ProvenanceService:
         )
         rows = (await self.session.execute(stmt)).all()
         post_ids = [ip.post_id for ip, _p, _s, _a in rows]
-        attachments = await self._attachments_for_posts(post_ids)
+        # 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
+        )
+        attachments = await self._attachments_for_posts(attach_post_ids)
         return {
             "image_id": image_id,
             "provenance": [