Serve feeds as text/xml instead of application/rss+xml

Browsers (Firefox in particular) force-download application/rss+xml and
application/atom+xml instead of rendering them, which is confusing for
site visitors who click a feed link directly rather than pasting it into
a reader. text/xml renders inline as an XML tree in every major browser.
Feed readers (FreshRSS included) sniff the actual XML content to detect
RSS/Atom, so this has no effect on subscription behavior.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

php/common.php

@@ -54,7 +54,12 @@ function http_get(string $url, array $extraHeaders = []): string
  */
 function serve_cached_feed(string $cacheKey, callable $builder): void
 {
-    header('Content-Type: application/rss+xml; charset=utf-8');
+    // text/xml plutot que application/rss+xml : les navigateurs affichent
+    // ce type en ligne (arbre XML) au lieu de forcer un telechargement,
+    // ce qui est plus rassurant pour un visiteur qui clique le lien
+    // directement depuis le site. FreshRSS et les autres lecteurs RSS
+    // detectent le format en lisant le contenu, pas ce header.
+    header('Content-Type: text/xml; charset=utf-8');
 
     if (!is_dir(CACHE_DIR)) {
         mkdir(CACHE_DIR, 0775, true);