feat: add admin panel with Discord OAuth and dashboard

Adds a React admin panel (panel/) with Discord OAuth2 login,
live dashboard via WebSocket, and settings/management pages.
Includes Docker build support, Vite proxy config for dev,
game_settings migration, and open-redirect protection on auth callback.

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

docker-compose.yml

@@ -47,6 +47,10 @@ services:
       - DISCORD_GUILD_ID=${DISCORD_GUILD_ID}
       - DISCORD_CLIENT_ID=${DISCORD_CLIENT_ID}
       - DATABASE_URL=postgresql://${DB_USER}:${DB_PASSWORD}@db:5432/${DB_NAME}
+      - DISCORD_CLIENT_SECRET=${DISCORD_CLIENT_SECRET}
+      - SESSION_SECRET=${SESSION_SECRET}
+      - ADMIN_USER_IDS=${ADMIN_USER_IDS}
+      - PANEL_BASE_URL=${PANEL_BASE_URL:-http://localhost:3000}
     depends_on:
       db:
         condition: service_healthy