fix(slice5): validate raw metadata input + prune stale bulk selections

Code-review follow-ups:
- Validate the raw submitted value before apply_values coerces it, in both
  the single-edit and bulk endpoints — a scalar set onto a tags field now
  rejects (422 / rejected) instead of silently char-splitting into a list.
- Catalog prunes bulk selections to the currently-visible (filtered) entries
  after each list fetch, so the bulk bar can't act on rows the user has
  filtered out of view.

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

frontend/src/components/Catalog.test.jsx

@@ -90,6 +90,30 @@ describe('Catalog faceted pane', () => {
     })
   })
 
+  it('drops a selection when a filter change hides the row', async () => {
+    getCollection.mockResolvedValue({
+      fields: { priority: { type: 'enum', values: ['P0', 'P1'] } },
+      viewer: { can_contribute: true }, entry_noun: 'RFC',
+    })
+    // Content-driven: unfiltered → row "a"; once the P0 facet is selected the
+    // server response omits "a" (drives the prune). Robust to extra mount-time
+    // fetches the collection effect triggers.
+    listRFCs.mockImplementation((p, c, opts) => Promise.resolve({
+      items: opts?.selections?.priority?.includes('P0')
+        ? [{ slug: 'b', title: 'B', state: 'active', tags: [], starred_by_me: false }]
+        : [{ slug: 'a', title: 'A', state: 'active', tags: [], starred_by_me: false }],
+      facets: { priority: { P0: 1 }, state: { active: 1 } },
+    }))
+    const { container } = renderCatalog()
+    fireEvent.click(await screen.findByLabelText(/select A/i))
+    expect(await screen.findByText(/1 selected/i)).toBeInTheDocument()
+    // Toggle the P0 facet checkbox → re-fetch returns a list without "a".
+    fireEvent.click(container.querySelector('.facet-group input[type="checkbox"]'))
+    await waitFor(() => {
+      expect(screen.queryByText(/1 selected/i)).not.toBeInTheDocument()
+    })
+  })
+
   it('does not offer row selection for a non-contributor', async () => {
     getCollection.mockResolvedValue({
       fields: { priority: { type: 'enum', values: ['P0'] } },