refactor: use inline basicauth for Traefik dashboard

Changed dashboard authentication to use inline basicauth
like Scrapy instead of external .htpasswd file:

**Changes:**
- Updated proxy labels to use basicauth.users=${PROXY_AUTH_USERS}
- Removed .htpasswd file and auth directory
- Removed dashboard-auth middleware from security.yaml
- Removed .htpasswd volume mount from compose.yaml

**Benefits:**
- Consistent with Scrapy authentication pattern
- Simpler configuration (no external files)
- Auth credentials managed centrally via .env

Dashboard accessible at: https://proxy.pivoine.art
Credentials: valknar / ragnarok98 (via PROXY_AUTH_USERS)

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

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

proxy/dynamic/security.yaml

@@ -59,8 +59,3 @@ http:
         average: 30
         burst: 15
         period: 1s
-
-    # Basic Auth for Traefik Dashboard
-    dashboard-auth:
-      basicAuth:
-        usersFile: /etc/traefik/.htpasswd