Implement initial Flutter app, secure backend, and legal templates

[mgabrielramos]

Motivation

• Provide a minimal, runnable scaffold for a production-ready stack (mobile client + secure backend) so the prototype can be iterated toward Play Store release.
• Keep API keys and heavy work server-side and introduce basic abuse protections (rate limiting, per-user daily limits).
• Add legal templates to meet Play Store / LGPD requirements and document how to run the new services.

Description

• Added a Node.js backend scaffold under backend/ including package.json, .env.example, README.md and src/server.js implementing Express, helmet, CORS, morgan, express-rate-limit, JWT auth, input validation (Zod), an in-memory usage store and endpoints POST /v1/auth/anonymous, POST /v1/translate and GET /health, with a stubbed translateWithGemini when GEMINI_API_KEY is not set.
• Added a Flutter app skeleton under mobile/ with pubspec.yaml and lib/main.dart implementing the main UI, a TranslationController using provider, demo translations, language selection UI, usage display and placeholders to integrate with the backend.
• Added legal templates legal/PRIVACY_POLICY.md and legal/TERMS_OF_USE.md and created productization guides under docs/ (FLUTTER.md, BACKEND.md, LEGAL.md, MONETIZATION.md, ROADMAP.md) to guide architecture, monetization and compliance.
• Updated README.md to describe the new folders and include quick run instructions for the backend (cd backend, cp .env.example .env, npm install, npm run dev).

Testing

• No automated tests were run for this change (scaffolding and documentation only).

---

Codex Task