Expose a reusable downstream app consumer contract

[vitormattos]

Summary

Add a generic downstream-app contract so Nextcloud app repositories can reuse this environment without copying its Compose topology.

Changes

• add APP_SOURCE, APP_ID and optional APP_SETUP_HOOK to the worker lifecycle;
• mount the downstream checkout into both Nextcloud and nginx through a reusable Compose fragment;
• keep the app path stable at /var/www/html/apps-extra/<APP_ID>;
• run an optional setup hook as the mapped www-data user after Nextcloud is ready;
• keep worker/project naming, mutable state, database selection, logs and teardown owned by the existing lifecycle;
• document a minimal Compose include adapter for devcontainers instead of duplicating the infrastructure topology;
• add contract tests for invalid identifiers/hook traversal;
• add an integration test with two consumers using the same logical app path, different source checkouts and independent teardown.

Compatibility

Existing workers that do not set APP_SOURCE behave exactly as before.

The contract remains generic to Nextcloud app development and contains no LibreSign-specific setup logic.

Validation

• commits created through the signed Git MCP;
• DCO trailers match the configured identity;
• GitHub reports the OpenPGP signatures as verified;
• CI will validate the existing database worker matrix plus the new downstream-consumer integration.

Closes #143

[vitormattos]

Superseded by #148. The original downstream-app contract made the worker aware of a primary app (APP_ID / APP_SOURCE) and duplicated responsibilities already represented by apps-extra. #148 has been rewritten around a generic Compose extension contract instead, which supports multiple apps and project-specific services without adding an app-specific lifecycle to NCDD.