Switch to official php:8.5.8-fpm-trixie image, fix extension/FPM build issues

Rebuild on the official PHP image instead of Sury's repo on bare Debian.
Fixes surfaced along the way: missing pkg-config/libonig-dev broke the
zip/mbstring extension builds, purging libzip-dev also auto-removed the
libzip5 runtime lib the compiled zip.so needs, phpredis needed bumping to
6.3.0 for PHP 8.5 header compatibility, and the php-fpm.conf listen
directive sed didn't match the commented-out default line so FPM never
created its unix socket, causing 503s from Apache's proxy_fcgi handler.

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

000-default.conf

@@ -13,10 +13,11 @@
 
     # PHP-FPM handler
     <FilesMatch \.php$>
-        SetHandler "proxy:unix:/run/php/php8.5-fpm.sock|fcgi://localhost/"
+        SetHandler "proxy:unix:/run/php/php-fpm.sock|fcgi://localhost/"
     </FilesMatch>
 
     # Docker-friendly logging
     ErrorLog /proc/self/fd/2
     CustomLog /proc/self/fd/1 combined
+
 </VirtualHost>