feat(storage): add MinIO to filesystem migration tool

Standalone binary (cmd/migrate-storage) that downloads all files from
MinIO and writes them to the local filesystem for decommissioning MinIO.

Queries revision files, item file attachments, and item thumbnails from
the database, then downloads each from MinIO preserving the object key
structure as filesystem paths. Supports --dry-run, --verbose, atomic
writes via temp+rename, and idempotent re-runs (skips existing files
with matching size).

.gitignore

@@ -1,6 +1,7 @@
 # Binaries
 /silo
 /silod
+/migrate-storage
 *.exe
 *.dll
 *.so