Increment 1 - Step 5: Stubs across the stack & Qt uniformity

[angelobarbu]

Step 5. Stubs across the stack

Partial end-to-end architecture: a running REST server, a QML client that talks to it, and the shared libraries between them.

What's new

• modulo_core — Result (on std::expected, stable dotted error codes) and version() injected from the CMake project version.
• modulo_api — HealthResponse and the uniform ErrorResponse envelope as Q_GADGET DTOs with validating QJson mappings (api::json::require* — no silent defaults).
• Server modules config (env → Config, validated) and http (QHttpServer, loopback-only binding, GET /api/v1/health, JSON 404 envelope), plus the modulo_server executable.
• modulo_client — ApiClient (QML-exposed, polls health every 3 s) and a dark Main.qml placeholder shell with a live status indicator.
• docs/high_level_design.md — Mermaid architecture diagrams (components, library graph, runtime flows).

Decisions taken along the way

• Full-Qt uniformity: QJson as the wire format (nlohmann-json removed), QString/.arg() project-wide, quint16-style typedefs, qInfo/qCritical in apps. The only Qt-free zone is modules/db + modulo_migrate.
• No [[nodiscard]] (clang-tidy check suppressed accordingly).

Toolkit/build fixes

• modulo_add_qml_app added; project version injection; generated qt.conf beside every executable (a second Homebrew Qt was shadowing the plugin path); WrapOpenGL_AGL pinned (newest macOS SDK dropped AGL); QML registrar basename-include handling.

Verified: clean -Werror build, format.sh --check, curl /api/v1/health → {"status":"ok","version":"0.1.0"}, 404 envelope, client running against the live server.