feat(database): migrate from SQLite to PostgreSQL and switch to Redis infrastructure- replace sqlite3 with PostgreSQL database adapter in Gemfile and config

• configure Redis for caching, Action Cable, and Active Job instead of Solid adapters
• add guide attribute to User model for onboarding flow control
• implement user existence validation in ApplicationController
• refactor CSS architecture by moving stylesheets to components directory
• create shared designs.css stylesheet for design pages with responsive styling
• update todo partial to check guide flag before displaying onboarding prompts
• add migration for guide column on users table