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>

frontend/src/components/modal/ProvenancePanel.vue

@@ -70,14 +70,19 @@
     </div>
 
     <div v-if="attachments.length" class="fc-prov__attach">
-      <h4 class="fc-prov__attach-title">Attachments</h4>
-      <a
-        v-for="at in attachments" :key="at.id"
-        class="fc-prov__attach-row"
-        :href="at.download_url" :download="at.original_filename"
-      >⬇ {{ at.original_filename }}
-        <span class="fc-prov__attach-size">({{ at.size_bytes }} B)</span>
-      </a>
+      <h4 class="fc-prov__attach-title">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. -->
+      <div class="fc-prov__attach-list">
+        <a
+          v-for="at in attachments" :key="at.id"
+          class="fc-prov__attach-row"
+          :href="at.download_url" :download="at.original_filename"
+        >⬇ {{ at.original_filename }}
+          <span class="fc-prov__attach-size">({{ at.size_bytes }} B)</span>
+        </a>
+      </div>
     </div>
   </section>
 </template>
@@ -229,6 +234,15 @@ function openPost(postId, artistId) {
   font-size: 13px; color: rgb(var(--v-theme-on-surface-variant));
   margin: 8px 0 4px;
 }
+.fc-prov__attach-list {
+  /* ~7 rows visible before scrolling; the clipped row hints at more.
+     Hairline scrollbar matching .fc-prov__cards. */
+  max-height: 180px;
+  overflow-y: auto;
+  scrollbar-width: thin;
+  scrollbar-color: rgb(var(--v-theme-surface-light)) transparent;
+  padding-right: 4px;
+}
 .fc-prov__attach-row {
   display: block; font-size: 13px; text-decoration: none;
   color: rgb(var(--v-theme-accent)); padding: 2px 0;