add-setting-env

Define the variable with zod validation in packages/env/src using createEnv from @t3-oss/env-core, then wire it through the server config type, global config assembly, and user store merge. Finally document it in .env.example and the self-hosting docs.

The priority is User Custom first, then Server Environment Variable, then Hardcoded Default. A user-configured value always overrides the server env var, which in turn overrides the built-in default.

Use zod schemas inside createEnv, for example z.coerce.number().min(1).max(20).optional() to coerce string env values into validated numbers with range constraints. Invalid values fail validation at config load time.

Prefer reusing existing types from packages/types/src/user/settings. Only add a new entry to GlobalServerConfig in packages/types/src/serverConfig.ts when introducing an entirely new settings domain.

Update the env definition in packages/env/src, GlobalServerConfig in packages/types, getServerGlobalConfig in apps/server, the user store action in src/store/user, .env.example, and both English and Chinese docs under docs/self-hosting/environment-variables.