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/api/file_handlers.go

@@ -21,7 +21,7 @@ type presignUploadRequest struct {
 	Size        int64  `json:"size"`
 }
 
-// HandlePresignUpload generates a presigned PUT URL for direct browser upload to MinIO.
+// HandlePresignUpload generates a presigned PUT URL for direct browser upload.
 func (s *Server) HandlePresignUpload(w http.ResponseWriter, r *http.Request) {
 	if s.storage == nil {
 		writeError(w, http.StatusServiceUnavailable, "storage_unavailable", "File storage not configured")
@@ -317,12 +317,9 @@ func (s *Server) HandleSetItemThumbnail(w http.ResponseWriter, r *http.Request)
 	w.WriteHeader(http.StatusNoContent)
 }
 
-// storageBackend returns the configured storage backend name, defaulting to "minio".
+// storageBackend returns the configured storage backend name.
 func (s *Server) storageBackend() string {
-	if s.cfg != nil && s.cfg.Storage.Backend != "" {
-		return s.cfg.Storage.Backend
-	}
-	return "minio"
+	return "filesystem"
 }
 
 // HandleUploadItemFile accepts a multipart file upload and stores it as an item attachment.