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.example

@@ -15,6 +15,7 @@ POSTGRES_DSN=postgresql+asyncpg://postgres:postgres@localhost:5432/messenger
 REDIS_URL=redis://localhost:6379/0
 
 S3_ENDPOINT_URL=http://localhost:9000
+S3_PUBLIC_ENDPOINT_URL=http://localhost:9000
 S3_ACCESS_KEY=minioadmin
 S3_SECRET_KEY=minioadmin
 S3_REGION=us-east-1