Update README.md to reflect modern Node.js stack

The README.md contains outdated information from the legacy Python/Heroku era that needs updating.

Outdated sections identified:

• References to Python tests (pytest) - should be removed
• PostgreSQL setup instructions (lines 58-65) - replaced with MongoDB
• Gunicorn server instructions (line 41) - replaced with Node.js/Express
• Heroku hosting reference (line 100) - migrated to Dokku
• Mixed technology references that no longer apply

Context:

• Complete migration from Python/Flask to Node.js/Express completed
• Database migrated from PostgreSQL/MariaDB to MongoDB
• Infrastructure migrated from Heroku to Dokku
• Build system migrated from Webpack to Vite

Acceptance Criteria:

• Remove outdated Python/PostgreSQL references
• Update setup instructions for Node.js/MongoDB stack
• Correct hosting information (Dokku instead of Heroku)
• Update testing instructions to reflect current test suite
• Ensure all technical information matches current architecture
• Verify all commands and instructions work with current setup

Priority: Medium

Start with README.md, especially the Gunicorn instruction at line 41, PostgreSQL setup at lines 58-65, and Heroku reference at line 100. Compare each section with the current Node.js/Express, MongoDB, Dokku, and Vite architecture and testing setup. Done means outdated references are removed, setup and testing instructions are current, and all documented commands and technical details are verified.