[core] Build the default application lazily

[frankrousseau]

Problem

• Importing zou.app or any submodule built the whole application as a side effect: CLI commands, RQ worker settings and remote runners paid the full boot (Redis config sync, 592 routes, plugins) even when they only needed helpers or config
• create_app() existed but nothing could skip the import-time build, and the test suite received an app it never explicitly constructed

Solution

• Build the module-level app lazily through a PEP 562 getattr on first access: from zou.app import app and the gunicorn zou.app:app entry point keep working unchanged, while importing zou.app becomes side-effect free
• Stop importing app at module scope in services and utils (constants read from the config module, runtime access through a local import or current_app), so a service imported first can no longer trigger the build mid-import
• Make entry points (debug server, event stream, test conftest) call create_app() explicitly, and add a fresh-interpreter test asserting imports stay build-free