test(stream): verify recovery and backpressure

[yanpgwang]

Summary

• prove a replacement API process recovers the restart window through open-stream-then-list without replaying old events
• prove a bounded slow subscriber is dropped without delaying a healthy subscriber or the PostgreSQL ledger
• record the completed SSE recovery and backpressure evidence in the compatibility docs

Verification

• make test-service
• PostgreSQL/NATS recovery tests repeated 20 times
• go test -race ./...
• PostgreSQL/NATS live package under the race detector
• go vet ./...
• make lint LINT_BASE=origin/main
• documentation typecheck and build

Refs #58