Fix port conflict: use !reset tag to replace base ports, not append

Docker Compose merges port lists from multiple -f files by concatenation.
Without !reset, both 0.0.0.0:8000:8000 (base) and 127.0.0.1:8000:8000
(prod override) are applied simultaneously, causing "port already allocated".
!reset clears the list before applying prod-specific bindings (requires
Docker Compose v2.24+, available in current docker-compose-plugin via apt).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -10,13 +10,13 @@
 
 services:
   postgres:
-    ports: []
+    ports: !reset []
 
   redis:
-    ports: []
+    ports: !reset []
 
   api:
-    ports:
+    ports: !reset
       - "127.0.0.1:${API_PORT:-8000}:8000"
     environment:
       PYTHONUNBUFFERED: "1"
@@ -26,7 +26,7 @@ services:
       PYTHONUNBUFFERED: "1"
 
   web:
-    ports:
+    ports: !reset
       - "127.0.0.1:${WEB_PORT:-3000}:3000"
     environment:
       NEXT_PUBLIC_BASE_PATH: "/ppt-tool"