fix: use AUTH_USERS environment variable in VERT auth middleware

Properly configured AUTH_USERS environment variable for Traefik container
to use in the vert-sablier.yaml dynamic configuration via Go templating.

Changes:
- Added AUTH_USERS environment variable to proxy compose file
- Updated vert-auth middleware to use {{ env AUTH_USERS }} template
- Fixed environment syntax to use map format instead of list format

This keeps credentials secure in the .env file (not tracked in git) while
the tracked vert-sablier.yaml file only contains the template reference.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

proxy/compose.yaml

@@ -56,6 +56,9 @@ services:
       retries: 3
       start_period: 10s
 
+    environment:
+      AUTH_USERS: ${AUTH_USERS}
+
     networks:
       - compose_network
 

proxy/dynamic/vert-sablier.yaml

@@ -13,7 +13,7 @@ http:
     vert-auth:
       basicAuth:
         users:
-          - valknar:$apr1$mPI9lhER$CIz4BtA8cQULi4McVZxfz.
+          - "{{ env AUTH_USERS }}"
 
     vert-compress:
       compress: {}