fix: make media uploads work behind docker

- add S3_PUBLIC_ENDPOINT_URL for browser-reachable presigned urls

- support both public/internal file url validation

- configure MinIO bucket CORS in minio-init

- update env examples and docs

.env.docker.example

@@ -27,6 +27,8 @@ MINIO_API_PORT=9000
 MINIO_CONSOLE_PORT=9001
 S3_REGION=us-east-1
 S3_BUCKET_NAME=messenger-media
+S3_PUBLIC_ENDPOINT_URL=http://localhost:9000
+S3_CORS_ALLOW_ORIGIN=*
 S3_PRESIGN_EXPIRE_SECONDS=900
 MAX_UPLOAD_SIZE_BYTES=104857600