feat(db): PostgresTransaction.find_one_and_update for work-kernel CAS

[eldonm]

Summary

• Add PostgresTransaction.find_one_and_update on the held connection (SELECT … FOR UPDATE + upsert) so CAS and insert_if_absent share one public transaction.
• Unit + Postgres integration coverage for commit/rollback atomicity of transition + outbox-style inserts.
• Bump to 0.0.21.

Test plan

• pytest tests/db/test_postgres_transaction_cas.py
• JVSPATIAL_POSTGRES_TEST_DSN=… pytest tests/db/test_postgres_integration.py::TestPostgresTransactionAtomicOps
• CI green; publish 0.0.21 after merge

Made with Cursor

[github-actions]

Benchmark comparison

Threshold: ±25% (informational, does not block merge)

benchmark	baseline (s)	current (s)	delta	status
tests/benchmarks/test_deferred_save_benchmarks.py::test_bench_deferred_save_batched_100	0.027124	0.027017	-0.4%	OK
tests/benchmarks/test_deferred_save_benchmarks.py::test_bench_immediate_save_100	0.025936	0.026546	+2.4%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_batched_saves_500	0.481181	0.511261	+6.3%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_count_empty_query	1.035469	1.109399	+7.1%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_count_filtered	1.138170	1.191172	+4.7%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_find_filtered	0.874956	0.904351	+3.4%	OK
tests/benchmarks/test_jsondb_benchmarks.py::test_bench_jsondb_save_throughput	0.001628	0.001714	+5.3%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_empty	0.349256	0.367945	+5.4%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_fallback_via_regex	0.385497	0.383738	-0.5%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_count_pushdown	0.351821	0.360920	+2.6%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_find_fallback_via_regex	0.381254	0.404738	+6.2%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_find_pushdown	0.353870	0.332799	-6.0%	OK
tests/benchmarks/test_sqlite_benchmarks.py::test_bench_sqlite_sort_limit_pushdown	0.375982	0.382539	+1.7%	OK