Add built-in High Availability (HA) support for Nextcloud Server deployments

Is your feature request related to a problem? Please describe.
Running Nextcloud in production today requires building custom high availability (HA) stacks using external infrastructure (load balancers, clustered databases, distributed filesystems). While this works, it creates a steep barrier for organizations that want reliable uptime but lack deep sysadmin resources. Home and SMB users, in particular, struggle to set up redundancy and failover without resorting to complex third-party solutions. This would allow users to run Nextcloud across two computers for redundancy and shared hosting of data.

Describe the solution you'd like
I would like Nextcloud Server to provide first-class, built-in support for high availability. This could take the form of:

• Native support for multiple application nodes behind a load balancer.
• Failover and/or clustering support for core services (e.g., Redis, background jobs).
• Guidance or tooling to coordinate shared storage and database connections in a resilient way.
• Ideally, a configuration mode or installer option (“Enable HA”) that makes redundancy achievable without deep manual integration work.

Describe alternatives you've considered

• Global Scale (Enterprise) already addresses scaling and resilience at very large deployments, but it is enterprise-only and more complex than many users need.
• Community users today rely on manual solutions: e.g., MariaDB/Galera, GlusterFS/Ceph, and HAProxy. While technically feasible, this is beyond the reach of many small to medium deployments.
• Nextcloud All-In-One simplifies deployment but still lacks a built-in HA path; attempts with Keepalived or shared storage are fragile and unofficial.

Additional context
This feature would help bridge the gap between individual/home deployments and large enterprise Global Scale. It would reduce downtime risks, increase adoption in SMEs, and simplify running Nextcloud in modern containerized environments (Docker/Kubernetes) where HA expectations are the norm.

Begin by mapping how Nextcloud Server currently handles application nodes, Redis, background jobs, shared storage, and database connections, then compare that with the Global Scale and All-in-One approaches mentioned in the issue. A complete result would need a defined, supported HA mode or tooling path, including clear failover and deployment behavior for the target environments.