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/storage.go

@@ -194,11 +194,14 @@ type SourceMetadata struct {
 	ContentHash     string              `json:"content_hash,omitempty"`
 	StatusCode      int                 `json:"status_code"`
 	ContentType     string              `json:"content_type,omitempty"`
+	ContentLength   int64               `json:"content_length,omitempty"`
 	ResponseHeaders map[string][]string `json:"response_headers,omitempty"`
 	FetchedAt       int64               `json:"fetched_at"`
+	FetchDurationMs int64               `json:"fetch_duration_ms,omitempty"`
 	ExpiresAt       int64               `json:"expires_at,omitempty"`
 	ETag            string              `json:"etag,omitempty"`
 	LastModified    string              `json:"last_modified,omitempty"`
+	RemoteAddr      string              `json:"remote_addr,omitempty"`
 }
 
 // Store writes metadata to storage.