feat: concurrent manifest downloads in ListSnapshots

Replace serial getManifestSize() calls with bounded concurrent downloads
using errgroup. For each remote snapshot not in the local DB, manifest
downloads now run in parallel (up to 10 concurrent) instead of one at a
time.

Changes:
- Use errgroup with SetLimit(10) for bounded concurrency
- Collect remote-only snapshot IDs first, pre-add entries with zero size
- Download manifests concurrently, patch sizes from results
- Remove now-unused getManifestSize helper (logic inlined into goroutines)
- Promote golang.org/x/sync from indirect to direct dependency

closes #8

go.mod

@@ -24,6 +24,7 @@ require (
 	github.com/spf13/cobra v1.10.1
 	github.com/stretchr/testify v1.11.1
 	go.uber.org/fx v1.24.0
+	golang.org/x/sync v0.18.0
 	golang.org/x/term v0.37.0
 	gopkg.in/yaml.v3 v3.0.1
 	modernc.org/sqlite v1.38.0
@@ -266,7 +267,6 @@ require (
 	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect
 	golang.org/x/net v0.47.0 // indirect
 	golang.org/x/oauth2 v0.33.0 // indirect
-	golang.org/x/sync v0.18.0 // indirect
 	golang.org/x/sys v0.38.0 // indirect
 	golang.org/x/text v0.31.0 // indirect
 	golang.org/x/time v0.14.0 // indirect