fix: root → /platform/ redirect via Traefik file config

Docker-compose label escaping mangles regex patterns. Use a separate
Traefik dynamic config file instead — clean regex, proper 302 redirect.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/traefik-dynamic.yml

@@ -0,0 +1,17 @@
+http:
+  routers:
+    root-redirect:
+      rule: "Path(`/`)"
+      priority: 100
+      entryPoints:
+        - websecure
+      tls: {}
+      middlewares:
+        - root-to-platform
+      service: saas@docker
+  middlewares:
+    root-to-platform:
+      redirectRegex:
+        regex: "^(https?://[^/]+)/?$"
+        replacement: "${1}/platform/"
+        permanent: false