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

internal/modules/modules.go

@@ -50,7 +50,7 @@ type Registry struct {
 var builtinModules = []ModuleInfo{
 	{ID: Core, Name: "Core PDM", Description: "Items, revisions, files, BOM, search, import/export", Required: true, Version: "0.2"},
 	{ID: Schemas, Name: "Schemas", Description: "Part numbering schema parsing and segment management", Required: true},
-	{ID: Storage, Name: "Storage", Description: "MinIO/S3 file storage, presigned uploads", Required: true},
+	{ID: Storage, Name: "Storage", Description: "Filesystem storage", Required: true},
 	{ID: Auth, Name: "Authentication", Description: "Local, LDAP, OIDC authentication and RBAC", DefaultEnabled: true},
 	{ID: Projects, Name: "Projects", Description: "Project management and item tagging", DefaultEnabled: true},
 	{ID: Audit, Name: "Audit", Description: "Audit logging, completeness scoring", DefaultEnabled: true},