Stop tracking .env

It has been in the repository since the initial commit carrying the live
database host, user and password. Removing it from the index stops that
getting worse; the credentials are still in history and should be
rotated, which needs coordinating with everything that reads them.

Deployments should pass configuration as container environment rather
than shipping a file — a file on disk is one `git add -f` away from
being committed again.

Also closes the last untested path: a shopper registration published
over the broker rather than posted over HTTP. All three MQTT topics —
order, customer and health — have now been fired against the live
Mosquitto instance and acknowledged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -48,3 +48,11 @@ Thumbs.db
 *.mov
 *.wmv
 
+
+# Local configuration. Tracked until 2026-08-03, which put the database
+# credentials in this repository's history — removing it from the index stops
+# that getting worse, but the existing history still has them and the password
+# should be rotated.
+.env
+.env.*
+!.env.example