refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

backend/settings/production.yaml

@@ -0,0 +1,8 @@
+debug: false
+frontend_url: ""
+
+application:
+  name: "sta-prod"
+  protocol: https
+  host: 0.0.0.0
+  base_url: ""