Custom database schema support for self-hosted Postgres

You can now run Tuist Server’s primary Postgres database in a custom schema instead of public. Set the new postgresql.schema value in the Helm chart (or TUIST_DATABASE_SCHEMA directly) to configure the schema for the server, migration job, build processor, and xcresult processor pods. This helps self-hosted environments that require a dedicated application namespace for stricter database security policies. Existing installs keep the default public schema, so no data movement is needed.