refactor(storage): remove MinIO backend, filesystem-only storage

Remove the MinIO/S3 storage backend entirely. The filesystem backend is
fully implemented, already used in production, and a migrate-storage tool
exists for any remaining MinIO deployments to migrate beforehand.

Changes:
- Delete MinIO client implementation (internal/storage/storage.go)
- Delete migrate-storage tool (cmd/migrate-storage, scripts/migrate-storage.sh)
- Remove MinIO service, volumes, and env vars from all Docker Compose files
- Simplify StorageConfig: remove Endpoint, AccessKey, SecretKey, Bucket,
  UseSSL, Region fields; add SILO_STORAGE_ROOT_DIR env override
- Change all SQL COALESCE defaults from 'minio' to 'filesystem'
- Add migration 020 to update column defaults to 'filesystem'
- Remove minio-go/v7 dependency (go mod tidy)
- Update all config examples, setup scripts, docs, and tests

.env.example

@@ -5,10 +5,6 @@
 # PostgreSQL
 POSTGRES_PASSWORD=silodev
 
-# MinIO
-MINIO_ACCESS_KEY=silominio
-MINIO_SECRET_KEY=silominiosecret
-
 # OpenLDAP
 LDAP_ADMIN_PASSWORD=ldapadmin
 LDAP_USERS=siloadmin