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>

backend/tests/test_metadata_bulk_endpoint.py

@@ -119,6 +119,39 @@ def test_bulk_remove_tag(app_with_fake_gitea):
     assert _sidecar(fake, "a")["tags"] == ["y"]
 
 
+def test_bulk_set_scalar_on_tags_rejected(app_with_fake_gitea):
+    # A scalar `set` onto a tags field must reject, not char-split into a list.
+    app, fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        _set_fields({"tags": {"type": "tags"}})
+        _seed_legacy(fake, "a", tags=["x"])
+        _refresh()
+        _login_owner(client)
+        r = client.post(f"{BASE}/meta/bulk",
+                        json={"slugs": ["a"], "op": "set",
+                              "field": "tags", "value": "checkout"})
+        assert r.status_code == 200, r.text
+        assert r.json()["applied"] == []
+        assert len(r.json()["rejected"]) == 1
+        assert r.json()["committed"] is False
+        assert not _has_sidecar(fake, "a")
+
+
+def test_bulk_set_list_on_tags_ok(app_with_fake_gitea):
+    app, fake = app_with_fake_gitea
+    with TestClient(app) as client:
+        _set_fields({"tags": {"type": "tags"}})
+        _seed_legacy(fake, "a", tags=["x"])
+        _refresh()
+        _login_owner(client)
+        r = client.post(f"{BASE}/meta/bulk",
+                        json={"slugs": ["a"], "op": "set",
+                              "field": "tags", "value": ["x", "y"]})
+        assert r.status_code == 200, r.text
+        assert r.json()["applied"] == ["a"]
+    assert _sidecar(fake, "a")["tags"] == ["x", "y"]
+
+
 def test_bulk_add_remove_requires_tags_field(app_with_fake_gitea):
     app, fake = app_with_fake_gitea
     with TestClient(app) as client: