chore(security): ignore .dev.vars and untrack committed secret

The Cloudflare Workers .dev.vars file holding the Neon URL and
Better Auth secret was tracked and leaked. Untrack it now and
add a gitignore rule so it cannot slip back in. History scrub
and credential rotation follow.

Author: Aqib-Rime

.gitignore

@@ -10,6 +10,11 @@ node_modules
 .env.test.local
 .env.production.local
 
+# Cloudflare Workers local secrets — never commit
+.dev.vars
+.dev.vars.*
+!.dev.vars.example
+
 # Testing
 coverage