fix: properly configure Apache MPM Prefork with dedicated config file

- Create dedicated apache/mpm_prefork.conf to override default MPM settings
- Mount mpm_prefork.conf to /etc/apache2/mods-available/ to properly apply limits
- Remove MPM config from nextcloud.conf (was being ignored)
- Set ServerLimit and MaxRequestWorkers to 400 (up from 150 default)
- Configure optimized worker settings for better concurrency

This fixes the "server reached MaxRequestWorkers" error that was causing
sync failures and 404 errors by properly overriding Apache's default
configuration file instead of trying to set it in conf-enabled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apache/nextcloud.conf

@@ -19,16 +19,6 @@ KeepAlive On
 KeepAliveTimeout 300
 MaxKeepAliveRequests 200
 
-# Configuration MPM Prefork - Augmentation des workers
-<IfModule mpm_prefork_module>
-    ServerLimit              400
-    StartServers             10
-    MinSpareServers          10
-    MaxSpareServers          20
-    MaxRequestWorkers        400
-    MaxConnectionsPerChild   1000
-</IfModule>
-
 <Directory /var/www/html/>
     Options FollowSymLinks MultiViews
     AllowOverride All