refactor: improve initial setup and configuration

[mrdorianh]

Intention: Allow quick start deploy to begin prototyping without the need to set up all env vars related to auth, stripe, and resend.

• Update .env.example with better organization and documentation
• Add clear sections for required and optional environment variables
• Update Stripe integration configuration and related files
• Update env.mjs to enforce required environment variables
• Update auth.config.ts to bypass auth in development if provider is not configured
• Update README with clearer setup instructions

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