Project level default for Server side only flags

## Problem

Server side only is currently a per flag setting that defaults to off. Teams that run their flags primarily on the server have to remember to toggle it on for every new flag. A missed toggle silently exposes a flag that was meant to stay on the server to client SDKs, and there is no way to set the safe default once at the project level.

## Request

Add a project level setting that makes newly created flags default to Server side only, with the existing per flag toggle used to opt out when a flag genuinely needs to reach client SDKs.

## Expected behaviour

- New flags created in a project with this setting enabled default to Server side only.
- Existing flags are unaffected.
- The per flag toggle stays, so individual flags can opt out.
- The project setting itself defaults to off, so current behaviour is preserved for everyone who does not enable it.

## Context

Requested by an enterprise customer who runs their flags primarily on the server and wants safe defaults, so a forgotten toggle cannot accidentally expose a flag to client SDKs.