test(images): add runtime acceptance gate for app image

[vitormattos]

Closes #49.

This implements the rewritten image-focused acceptance test for the app image.

What it validates:

• the exact locally built image can start with an ephemeral PostgreSQL dependency;
• Nextcloud autoinstall completes;
• occ status --output=json reports installed: true;
• occ check passes;
• the FPM runtime accepts a real FastCGI request to status.php;
• diagnostics are printed on failure;
• all test-created containers, network, and client image tag are cleaned up.

The test is independent from GitHub Actions and accepts an already-built image reference. The existing build action now runs the same test against both scan/app:amd64 and scan/app:arm64 after Trivy. Publication therefore remains gated by build -> scan -> runtime acceptance.

No deployment Compose stack, Redis/web integration, Playwright, LibreSign-specific behavior, or image architecture refactor is introduced here.