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/cache.go

@@ -238,10 +238,13 @@ func (c *Cache) StoreSource(
 		Path:            req.SourcePath,
 		Query:           req.SourceQuery,
 		ContentHash:     contentHash,
-		StatusCode:      httpStatusOK,
+		StatusCode:      result.StatusCode,
 		ContentType:     result.ContentType,
+		ContentLength:   result.ContentLength,
 		ResponseHeaders: result.Headers,
 		FetchedAt:       time.Now().Unix(),
+		FetchDurationMs: result.FetchDurationMs,
+		RemoteAddr:      result.RemoteAddr,
 	}
 
 	if err := c.srcMetadata.Store(req.SourceHost, pathHash, meta); err != nil {