Add pagination, search, and sorting to admin sessions ...

Backend:
- Add count_all and search_list abstract methods to SessionStore
- Implement count_all and search_list in PgSessionStore (SQL with ILIKE)
- Implement count_all and search_list in InMemorySessionStore
- Update /admin/sessions to accept limit, offset, search, sort_by, sort_order
- Return {total, limit, offset, items} from /admin/sessions

Frontend:
- Add search input for sessions in admin panel
- Add clickable sortable column headers with asc/desc toggle
- Add pagination controls (prev/next, page size selector, item count)
- Debounce search input (300ms)

Tests:
- Add integration tests for pagination, offset, search, and sorting
- All 217 tests pass

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>