Show server-side progress during Git pushes #112

After Git finishes uploading a pack, our service waits silently while it imports objects, validates the update and publishes it. A real push of object-log exposed a noticeable pause after the client's Total ... line.

The local workload uploaded 6,282 objects, including 4,198 deltas, in a 2.78 MiB pack. It succeeded: the stored main matched the local branch. The missing feedback makes a working push appear stalled.

The service currently does not advertise receive-pack sideband progress. Upstream go-git provides PreReceiveInfo.Progress, but that hook runs after pack import, so merely adding a message there does not cover the whole pause.

Investigate the smallest way to provide standard Git progress during the expensive server phases, using existing go-git APIs where possible. Keep object-log itself independent of Git and Spin.

Acceptance:

• An ordinary Git client receives useful progress before a slow push finishes, including during import if the existing APIs allow it.
• Success is reported only after durable publication; failed and uncertain operations retain their current semantics.
• Quiet clients and clients without progress negotiation still work.
• Exercise the real object-log history against disposable local Spin/MinIO, alongside focused protocol/error tests. Keep added runtime code small.

Reproduce with the viewer's documented local launcher, then push this repository's main to a fresh carsonfarmer/object-log.git name. Coordinate measurements with the storage-amplification investigation so concurrent runs do not distort results.