feat(storage): add S3/R2 abstraction with local Rustfs dev

- Add backend/internal/s3/ with build-tag pattern (dev vs prod)
- Dev: Uses local Rustfs container (S3-compatible)
- Prod: Stub for R2 Cloudflare (add AWS SDK to implement)
- Add S3 env vars to .env.example and .env
- Add Rustfs service to compose.yml
- Add Rustfs reset to local-dev-2.sh (wipes data on each run)

compose.yml

@@ -59,19 +59,38 @@ services:
     ports:
       - "80:80"
       - "443:443"
+      - "9000:9000" # Rustfs S3 API
     volumes:
       - ./nginx/conf.d:/etc/nginx/conf.d
       - ./nginx/certs:/etc/nginx/certs
       - ./frontend/build:/usr/share/nginx/html # Serve frontend
       - ./sabredav:/var/www/dav
+      - rustfs_data:/data
     depends_on:
       - backend
       - sabredav
     networks:
       - appnet
 
+  rustfs:
+    image: rustfs/rustfs:latest
+    container_name: rustfs
+    restart: always
+    ports:
+      - "9001:9000" # Web console
+    environment:
+      - RUSTFS_ADMIN_ACCESS_KEY=minioadmin
+      - RUSTFS_ADMIN_SECRET_KEY=minioadmin
+    volumes:
+      - rustfs_data:/data
+    networks:
+      - appnet
+    command: >
+      bash -c "rustfs server /data --console-address :9000 --address :9000 --axum-server"
+
 volumes:
   pgdata:
+  rustfs_data:
 
 
 networks: