I built a custom AI assistant platform with Laravel + multi-provider LLMs + pgvector - here's what I learned

I've been a software engineer for a while. A few months ago I decided to build a SaaS product solo. This is a technical write-up of what I shipped, the architectural decisions I made, and the honest lessons from building it. The product is ChatNexus - a platform that lets any business deploy a custom AI assistant trained on their own data. Upload documents, paste text, crawl a URL, or sync a Google Doc - your assistant learns from your content and can answer questions, handle bookings, and run 24/7 on your website. Here's the full stack and why I chose each piece.