Fix 500 during contact search; add comments and format files

[ms]

Hello, I’m an early reviewer of the book.

A few minor fixes:

• Searching throws a 500 because some of the mock data has null values. Added a check for None
• Added a comment about the secret key so people don’t copy paste this line without thinking
• Ran autopep8 to pretty-format the code on app and contact_model (hopefully that’s the right thing to do, I know even less Python than you say you do)

The first item leads me to ask: how does htmx handle HTTP errors? I didn’t see any indication of something being wrong in the UI (I had to open the devtools and then look at the terminal output).

I only tested searching. I’m happy to split this PR up if you prefer to keep commits more atomic in the repo.

Loving the book so far, great work!