perf: Use INSERT RETURNING for event creation

[smoreinis]

Summary

Optimize the event write path by using PostgreSQL's RETURNING clause to get the auto-generated sequence_id in a single query instead of two.

Before (2 queries per event)

session.add(orm)
await session.commit()
await session.refresh(orm)  # Extra SELECT to get sequence_id

After (1 query per event)

stmt = insert(EventORM).values(...).returning(EventORM)
result = await session.execute(stmt)
orm = result.scalar_one()
await session.commit()

Benchmark Results

Tested with 200 iterations on docker-to-docker network:

Metric	OLD (add+refresh)	NEW (RETURNING)	Improvement
Mean latency	0.812ms	0.684ms	15.8% faster
Median	0.780ms	0.644ms	17.4% faster
P95	1.050ms	0.887ms	15.5% faster
P99	1.780ms	1.009ms	43.3% faster
Throughput	1,231 events/sec	1,462 events/sec	+231 events/sec

Note: Docker-to-docker latency is ~0.1ms. In production with typical network latency (1-5ms), the improvement would be more significant since we're eliminating an entire network round-trip.

Test plan

• All 20 event-related tests pass
• All 188 unit tests pass
• Verified sequence_id is correctly populated in returned entity
• Benchmarked locally with 200 iterations