📝 Fix README factual errors and fill documentation gaps - #6

Summary

• Correct stale facts: test count (153→160), main.py LOC (89→135), and the Project Structure tree (post-split frontend, backend tests/utils/prompts subdirs, root Docker/CI files).
• Add a GenAI Logic section covering the ReAct agent loop, the four @tool functions and when each is selected, the RAG pipeline (chunk → embed → retrieve → optional rerank → top-5 context, with the reranker-on-vs-off retrieval count made explicit), prompt caching, and file-based prompt versioning including both /admin/prompts endpoints.
• Surface the seeded admin login credentials in Quick Start, right after the Docker command, with a note that they're demo defaults to change before real deployment.
• Add an Auth column to the API Endpoints table, plus the missing /reports and /admin/prompts rows.
• Add missing env vars (SECRET_KEY, ACCESS_TOKEN_EXPIRE_MINUTES, FIRST_SUPERUSER_EMAIL/PASSWORD, INSTALL_RERANK, SENTRY_DSN) to the Environment Variables table.
• Reorder Quick Start so the one-command Docker path leads (moved up from the end of Architecture) with the manual venv/npm route as an alternative, and credentials/re-seeding after the app is actually running.
• Translate Roadmap Phase 5 from Greek to English for consistency with the rest of the document.

Test plan

• Render README.md on GitHub and confirm formatting/tables look right
• Click through all relative links and images (logo, screenshots, PDF, LICENSE, agent/executor.py, agent/tools.py, core/prompt_loader.py)
• Confirm the mermaid architecture diagram still renders