docs: update project readmes

[hank1101444]

No description provided.

[Copilot]

Pull request overview

Documentation-only refresh of the project's README files. The root README is restructured to highlight architecture, deployment modes (Docker, local dev, CI/K3s/Argo CD), tech stack, default accounts, environment variables, and feature/API entry points. The frontend and backend READMEs are rewritten from boilerplate/minimal content into structured guides covering tech stack, scripts, routes/modules, project layout, and env variables.

Changes:

• Rewrite root README.md into a structured overview with badges, architecture diagram, deployment modes, and feature/env tables.
• Replace frontend/README.md boilerplate with a Next.js-focused guide (scripts, routes, structure, UI conventions).
• Expand backend/README.md into a FastAPI guide covering API routes, env variables, migrations, tests, and Docker services.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Restructured top-level project overview with new sections for architecture, modes, env, and features.
frontend/README.md	Replaced create-next-app boilerplate with project-specific frontend documentation.
backend/README.md	Expanded backend docs with route registry, env vars, migrations, tests, and compose services.

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[ChuuuDwarf]

LGTM!