Retry failed GitHub release scans with backoff (#133)

Releasing a GitHub sync lease always advanced `last_synced_at`, even after a failed scan (e.g. a rate-limit 403). A remote that failed once waited a full `mutable_ttl` before retrying, and a cold remote kept returning 503 until then.

- record scan outcomes in one shared lease helper for github_rpm/deb/alpine
- keep `last_synced_at` and the ETag on failure; retry from 60s with exponential backoff, capped at min(10m, ttl/4)
- honour `Retry-After` / `X-RateLimit-Reset`, clamped to `mutable_ttl`
- add `sync_failures` / `next_retry_at` columns (migration 0002)

Reviewed-on: #133
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/database/deb_github_sync_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 	"time"
 
+	"git.unkin.net/unkin/artifactapi/internal/provider"
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
@@ -44,7 +45,7 @@ func TestGitHubDebSyncLease(t *testing.T) {
 		t.Fatal("replica-2 claimed while replica-1 holds the lease")
 	}
 
-	if err := testDB.ReleaseGitHubDebSyncLease(ctx(), name, "replica-1", `"etag-1"`, time.Now()); err != nil {
+	if err := testDB.ReleaseGitHubDebSyncLease(ctx(), name, "replica-1", provider.SyncResult{Etag: `"etag-1"`}); err != nil {
 		t.Fatalf("release: %v", err)
 	}