feat(images): replace version-specific app image foundation

[vitormattos]

Closes #66.
Blocks reconstruction of #62.
Part of #47.

This PR removes the Nextcloud-major-specific app image path and replaces it with one generic app Dockerfile.

What changes:

• .docker/app/Dockerfile now supports explicit release and daily source modes;
• the base runtime is configurable independently from the Nextcloud server payload;
• daily archives are verified against upstream SHA-512 before extraction;
• the PHP extension installer is pinned to release 2.12.0 and verified by SHA-256;
• the existing Compose environment keeps NEXTCLOUD_VERSION as its user-facing setting and maps it to the generic base image;
• .docker/app/Dockerfile.35 is removed;
• the old Nextcloud-35 publication workflow is removed;
• a new validation-only workflow builds Nextcloud Server master from the same Dockerfile for amd64 and arm64, scans both images, and runs the same Bats runtime acceptance gate introduced by Add runtime acceptance tests for the Nextcloud app image #49.

Intentionally not included:

• no :master-fpm publication yet;
• no new stable/full-version tagging workflow;
• no second Compose stack;
• no Redis/web integration expansion.

The final development publication workflow should be rebuilt only after this foundation is merged.