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

scripts/deploy.sh

@@ -8,7 +8,6 @@
 #   - SSH access to the target host
 #   - /etc/silo/silod.env must exist on target with credentials filled in
 #   - PostgreSQL reachable from target (set SILO_DB_HOST to override)
-#   - MinIO reachable from target (set SILO_MINIO_HOST to override)
 #
 # Environment variables:
 #   SILO_DEPLOY_TARGET  - target host (default: silo.example.internal)