Add document list pagination

[suguanYang]

Summary

• Add page and page_size query params to GET /v1/documents
• Return document-list pagination metadata matching the chunk-list response style
• Add a count query and paged document query ordered by updated_at desc, document_id asc
• Cover default and second-page behavior in document contract tests

Verification

• uv run pytest apps/api/tests/contract/test_documents_contract.py -q
• make lint
• make typecheck
• git diff --check