feat(gateway): API Key 인증 + HTTPS (Caddy) 보안 강화

- gateway.py: auth middleware — /api/* 엔드포인트에 Bearer token 필수
- Caddyfile: Let's Encrypt 자동 HTTPS 리버스 프록시
- docker-compose.yml: Caddy 추가, Gateway 포트 내부 전용
- config.py: GATEWAY_API_KEY 설정 추가
- .env: 키 생성 명령어 가이드 포함

docker-compose.yml

@@ -3,14 +3,15 @@ services:
     build: .
     container_name: gravity-gateway
     restart: unless-stopped
-    ports:
-      - "8585:8585"
+    # Port NOT exposed directly — Caddy handles external access
+    expose:
+      - "8585"
     environment:
       - DISCORD_TOKEN=${DISCORD_TOKEN}
       - DISCORD_GUILD_ID=${DISCORD_GUILD_ID}
       - BOT_MODE=gateway
       - GATEWAY_PORT=8585
-      # Brain path inside container (not used in gateway mode, but needed for config validation)
+      - GATEWAY_API_KEY=${GATEWAY_API_KEY}
       - BRAIN_PATH=/app/data/brain
     volumes:
       - gateway-data:/app/data
@@ -20,5 +21,21 @@ services:
         max-size: "10m"
         max-file: "3"
 
+  caddy:
+    image: caddy:2-alpine
+    container_name: gravity-caddy
+    restart: unless-stopped
+    ports:
+      - "443:443"
+      - "80:80"
+    volumes:
+      - ./Caddyfile:/etc/caddy/Caddyfile:ro
+      - caddy-data:/data
+      - caddy-config:/config
+    depends_on:
+      - gateway
+
 volumes:
   gateway-data:
+  caddy-data:
+  caddy-config: