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.

docsgpt/api/audit.py

@@ -14,6 +14,9 @@ Two properties matter and are enforced here rather than at every call site:
 2. **Request context is optional.** Celery tasks (ingestion finishing, a
    scheduled run) have no Flask request; the row simply records without an IP
    or user agent instead of raising.
+
+The event → category taxonomy the admin activity feed filters on lives in
+``docsgpt/audit_events.py``, which the storage layer needs too.
 """
 
 from __future__ import annotations
@@ -23,14 +26,10 @@ from typing import Any, Optional
 
 from flask import has_request_context, request
 
-from docsgpt.audit_events import ACTIVITY_CATEGORIES, category_for
 from docsgpt.storage.db.repositories.auth_events import AuthEventsRepository
 
 logger = logging.getLogger(__name__)
 
-# Re-exported so route modules have one import for recording and classifying.
-__all__ = ["ACTIVITY_CATEGORIES", "category_for", "record_event"]
-
 
 def record_event(
     conn,