fix(backend): serve original content with byte-range support

GetContent streamed the whole file with a plain 200/io.Copy and no
Accept-Ranges, so the browser couldn't seek or scrub audio/video opened
from the viewer. It now serves seekable bodies (the disk store returns an
*os.File) via http.ServeContent, which advertises Accept-Ranges and
answers Range requests with 206 Partial Content; non-seekable bodies
still fall back to a plain stream. Adds an integration test asserting a
ranged request returns 206 with the right Content-Range and bytes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/handler/file_handler.go

@@ -407,9 +407,20 @@ func (h *FileHandler) GetContent(c *gin.Context) {
 	if c.Query("inline") == "1" {
 		disposition = "inline"
 	}
+	name := ""
 	if res.OriginalName != nil {
+		name = *res.OriginalName
 		c.Header("Content-Disposition",
-			fmt.Sprintf("%s; filename=%q", disposition, *res.OriginalName))
+			fmt.Sprintf("%s; filename=%q", disposition, name))
+	}
+
+	// Serve with byte-range support when the body is seekable (it is for the
+	// disk store): http.ServeContent advertises Accept-Ranges and answers Range
+	// requests with 206 Partial Content, which is what lets the browser scrub and
+	// seek within audio/video. Fall back to a plain stream otherwise.
+	if seeker, ok := res.Body.(io.ReadSeeker); ok {
+		http.ServeContent(c.Writer, c.Request, name, time.Time{}, seeker)
+		return
 	}
 	c.Status(http.StatusOK)
 	io.Copy(c.Writer, res.Body) //nolint:errcheck