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/facebook.php

@@ -16,7 +16,8 @@ function fetch_facebook_feed(): string
     return http_get(FETCHRSS_URL, ['Accept: application/atom+xml, application/xml']);
 }
 
-header('Content-Type: application/atom+xml; charset=utf-8');
+// text/xml plutot que application/atom+xml : voir la note dans common.php
+header('Content-Type: text/xml; charset=utf-8');
 
 if (!is_dir(CACHE_DIR)) {
     mkdir(CACHE_DIR, 0775, true);