fix: make local docker uploads replica-independent (#104)

## Why

Chunked blob uploads kept the in-progress session in **process memory** keyed by upload UUID, so the `POST`/`PATCH`/`PUT` of a single `docker push` had to land on the same replica. The API runs at `minReplicas: 2` with no session affinity (see argocd-apps `api-hpa.yaml`), so a real push — which streams the layer via `PATCH` then finalises with `PUT` — intermittently 404s with `BLOB_UPLOAD_UNKNOWN` when a chunk hits a replica that never saw the `POST`. This was flagged when the local docker registry landed (#103).

## Changes

- Stage chunked uploads in object storage under `uploads/<uuid>` instead of an in-memory temp file. The UUID travels in the `Location` URL handed to the client, so any replica reconstructs the staging key with no shared in-process state. Finalise streams the staged bytes plus any trailing `PUT` body through the CAS in one pass; monolithic uploads are unchanged.
- Support `DELETE` of an in-progress upload (cancel) by dropping its staging object.
- Reap abandoned staging objects in the GC (`uploads/` older than 24h) via a new `S3.ListStaleObjects`, so cancelled/interrupted pushes don't leak.

## Verification

- Split a single push across **two instances sharing one Postgres+MinIO**: `POST`→A, `PATCH`→B, `PUT`→A finalises with the correct digest, and the blob pulls back **byte-identical from both** replicas. Config-blob and manifest pushes split the same way succeed; `tags/list` is correct. (Pre-fix, the cross-replica `PATCH` 404s.)
- `scripts/docker-e2e.sh` still passes (incl. `TestLocalDockerPushPull`); unit tests + `go vet` clean.

Reviewed-on: #104
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/api/v1/proxy.go

@@ -24,7 +24,6 @@ type ProxyHandler struct {
 	store         *storage.S3
 	local         *v2.LocalHandler
 	cas           *storage.CAS
-	uploads       *uploadStore
 }
 
 func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *database.DB, store *storage.S3, local *v2.LocalHandler) *ProxyHandler {
@@ -35,7 +34,6 @@ func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *da
 		store:         store,
 		local:         local,
 		cas:           storage.NewCAS(store),
-		uploads:       newUploadStore(),
 	}
 }