Add fetch/conversion metrics and improve logging

FetchResult now includes:
- StatusCode: HTTP status from upstream
- FetchDurationMs: time to fetch from upstream
- RemoteAddr: upstream server address

SourceMetadata now stores:
- ContentLength: size from upstream
- FetchDurationMs: fetch timing
- RemoteAddr: for debugging

Image conversion log now includes:
- host: source hostname (was missing)
- path: source path (renamed from file)
- convert_ms: image processing time
- quality: requested quality setting
- fit: requested fit mode

internal/imgcache/imgcache.go

@@ -178,6 +178,12 @@ type FetchResult struct {
 	ContentType string
 	// Headers contains all response headers from upstream
 	Headers map[string][]string
+	// StatusCode is the HTTP status code from upstream
+	StatusCode int
+	// FetchDurationMs is how long the fetch took in milliseconds
+	FetchDurationMs int64
+	// RemoteAddr is the IP:port of the upstream server
+	RemoteAddr string
 }
 
 // Processor handles image transformation (resize, format conversion)