fix(admin): keyset export paging, rollup-free spend, consistent totals

Review pass over the preceding commits.

- The activity export paged by OFFSET. The journals are append-only and the
  feed is newest-first, so rows written mid-export shift the window down and
  repeat rows already emitted. Pages by keyset on (created_at, feed, id) now.
- top_token_users and the per-user breakdown counted run-level rollup rows.
  A scheduled run already has a row per LLM call, so its spend was billed
  twice -- invisible while the column was tokens, obvious once it was dollars.
  Both now exclude them, matching every other spend query.
- total_cost was summed from the per-model split, which drops rows with no
  model_id and so undercounted the figure printed above the series. Summed
  from the series instead.
- The CSV export encoded its detail cell without the fallback the NDJSON
  branch had, so a non-JSON-native value would have failed the stream.
- The activity view reset pagination in an effect, which fetched the stale
  page against the new filters before fetching again. Reset in the setters.
- The audit taxonomy is no longer re-exported from the helper module; the one
  caller that wanted it imports from where it lives.

tests/api/test_audit_helper.py

@@ -6,7 +6,8 @@ from unittest.mock import MagicMock, Mock, patch
 
 import pytest
 
-from docsgpt.api.audit import ACTIVITY_CATEGORIES, category_for, record_event
+from docsgpt.api.audit import record_event
+from docsgpt.audit_events import ACTIVITY_CATEGORIES, category_for
 
 
 @pytest.fixture