feat: track download counts for both cache hits and cache misses

Previously, download counts only incremented on cache hits (when package
was served from cache). First-time downloads (cache misses) were not counted.

Changes:
- Add UpdateDownloadCount() call when serving newly cached packages
- This ensures every download through the proxy increments the counter
- Analytics tracking also added for cache misses

Behavior now:
- First download (cache miss): counter = 1
- Second download (cache hit): counter = 2
- Third download (cache hit): counter = 3
- etc.

Updated all relevant tests to expect the additional UpdateDownloadCount call.

Resolves user requirement: "I want the counters to increase whenever
package is downloaded via proxy - regardless of it being new download
or cached download"

pkg/cache/cache.go

@@ -320,6 +320,22 @@ servePkg:
 		return nil, errors.Wrap(err, errors.ErrCodeStorageFailure, "failed to retrieve just-stored package")
 	}
 
+	// Track download count for first-time download (cache miss)
+	// This ensures download count increments regardless of cache hit/miss
+	if err := m.metadata.UpdateDownloadCount(ctx, registry, name, version); err != nil {
+		log.Warn().
+			Err(err).
+			Str("registry", registry).
+			Str("package", name).
+			Str("version", version).
+			Msg("Failed to update download count for newly cached package")
+	}
+
+	// Track download in analytics if enabled
+	if m.analytics != nil {
+		m.trackDownload(registry, name, version, storedPkg.Size)
+	}
+
 	return &CacheEntry{
 		Package:     storedPkg,
 		Data:        storedData,