Add SQLite as a first-class worker backend

[vitormattos]

Summary

Implement the SQLite branch of the worker contract introduced by #140/#144.

Changes

• install pdo_sqlite in all supported PHP development images;
• allow DB_TYPE=sqlite in dev-worker;
• install Nextcloud with the SQLite driver;
• avoid starting the network database service for SQLite workers;
• keep the SQLite database inside each worker's isolated Nextcloud filesystem;
• add worker contract coverage for SQLite;
• add a real CI integration job that builds the PHP 8.3 image, starts two SQLite workers, verifies pdo_sqlite, writes conflicting Nextcloud-visible state and confirms isolation;
• document the non-interactive SQLite lifecycle.

Compatibility

MySQL remains the default. PostgreSQL and the legacy DB_HOST=pgsql path remain supported.

MariaDB remains intentionally reserved for #142.

Validation

• commit created through the signed Git MCP;
• DCO trailer matches the configured identity;
• GitHub reports the OpenPGP signature as verified;
• CI will exercise Compose rendering, shell checks, image build and the two-worker SQLite integration.

Closes #141