Add isolated worker lifecycle foundation

[vitormattos]

Summary

Introduce the foundation required by #140 before adding the SQLite and MariaDB-specific layers from #141 and #142.

This PR does not close #140 yet.

Changes

• separate database backend selection (DB_TYPE) from the database connection hostname (DB_HOST);
• preserve the existing DB_HOST=pgsql docker compose up compatibility path;
• parameterize mutable bind-mount state with WORKER_VOLUMES_DIR;
• add a deterministic dev-worker lifecycle for MySQL/PostgreSQL workers;
• isolate each worker under .workers/<worker-id>/volumes and a dedicated Compose project;
• provide up, exec, logs, config, status and destroy operations;
• add Bats contract tests and include them in the existing unit-test workflow;
• document the new worker contract and the current backend boundary.

SQLite and MariaDB are deliberately rejected by this foundation helper until their dedicated implementations land, so automation cannot accidentally start a partial environment and report it as supported.

Compatibility

The Compose interpolation keeps MySQL as the default and preserves legacy PostgreSQL selection through DB_HOST=pgsql.

Validation

• commits created with DCO and GitHub-verified OpenPGP signatures;
• CI exercises Compose rendering and the worker contract;
• full concurrent runtime/isolation tests remain part of Add an automated isolated test-worker lifecycle with full database matrix support #140/Add SQLite as a first-class database backend #141/Add MariaDB as a first-class database backend #142 follow-up work.

Part of #140