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

README.md

@@ -34,7 +34,7 @@ silo/
 │   ├── ods/               # ODS spreadsheet library
 │   ├── partnum/           # Part number generation
 │   ├── schema/            # YAML schema parsing
-│   ├── storage/           # MinIO file storage
+│   ├── storage/           # Filesystem storage
 │   └── testutil/          # Test helpers
 ├── web/                   # React SPA (Vite + TypeScript)
 │   └── src/
@@ -55,7 +55,7 @@ silo/
 
 See the **[Installation Guide](docs/INSTALL.md)** for complete setup instructions.
 
-**Docker Compose (quickest — includes PostgreSQL, MinIO, OpenLDAP, and Silo):**
+**Docker Compose (quickest — includes PostgreSQL, OpenLDAP, and Silo):**
 
 ```bash
 ./scripts/setup-docker.sh
@@ -65,7 +65,7 @@ docker compose -f deployments/docker-compose.allinone.yaml up -d
 **Development (local Go + Docker services):**
 
 ```bash
-make docker-up    # Start PostgreSQL + MinIO in Docker
+make docker-up    # Start PostgreSQL in Docker
 make run          # Run silo locally with Go
 ```