Describe the bug

When a user changes their username, their old public DevCard URL (e.g. devcard.app/oldusername) returns a 404 instead of redirecting to the new URL. Any previously shared links or QR codes pointing to the old URL break permanently.

Steps to Reproduce

1. Create a DevCard with username alice
2. Share or embed the card URL
3. Change username to alice2
4. Visit the old URL — 404 error

Expected Behavior

The old username URL should 301 redirect to the new URL for at least 90 days.

Suggested Fix

Store previous usernames in a username_redirects table and add a redirect middleware that checks it before returning 404.