Fix waitGroup.Wait() blocking forever when Add is never called

Add documentation for *WithDB constructors

Add tests for *WithDB constructors

Add *WithDB constructors for SQL backends

Remove resolved TODO in tester.go scheduleSubWorkflow

Remove resolved TODO comment in type_test.go

Update Agents.md

Bump github.com/jackc/pgx/v5 from 5.3.1 to 5.5.4

Fix GitHub URL in postgres README (remove www prefix)

Add PostgreSQL backend documentation following SQLite/MySQL pattern

fix(samples): correct database creation query syntax

refactor: rename package from postgresbackend to postgres

fix: update makefile

migrations: squash migrations into a single file

fix: lint errors