Fix Traefik/Docker API version mismatch, tidy Matrix registration-token env handling

docker-compose.yml

@@ -1,6 +1,8 @@
 services:
   traefik:
-    image: traefik:v3.1
+    # Must be >=3.6 — Docker 29 raised its minimum API version to 1.44, and Traefik's
+    # Docker provider only gained version auto-negotiation in v3.6.
+    image: traefik:v3.6
     container_name: traefik
     restart: unless-stopped
     command:
@@ -43,6 +45,10 @@ services:
       # (see README "First boot: Matrix bot account").
       CONTINUWUITY_ALLOW_FEDERATION: "false"
       CONTINUWUITY_ALLOW_REGISTRATION: ${MATRIX_ALLOW_REGISTRATION:-false}
+      # CONTINUWUITY_REGISTRATION_TOKEN is deliberately NOT set here: Continuwuity treats
+      # a present-but-empty value as a config error (fails to start), so it can only be
+      # added here temporarily (with a real value) when you need to register a new user,
+      # then removed again — see README "Adding another Matrix user".
     volumes:
       - matrix_data:/var/lib/continuwuity
     networks: