List all system settings
Return every row in the system_settings table (system-scope, not tenant-scope). SystemAdmin only.
Response
list of settings
Category groups settings in the management UI ("limits", "agent", "auth", ...). Free-form string so adding a new category is a data-only change.
Enum is populated by the service layer (NOT persisted) from the in-code registry. Empty/nil means "free-form input"; non-empty means the UI should render a select with these options. Tagged gorm:"-" so GORM never tries to read/write the column.
IsSecret reserves UI affordances for P3 (mask + reveal-with-confirm). In P1 every row is is_secret=false; service Update accepts the column but does not yet enforce special handling.
LastModifiedByName is a display label resolved from LastModifiedBy (the user's UUID) at handler time — username when available, otherwise email. Empty for virtual rows that were never persisted (LastModifiedBy=""). Not stored: derived per request so renaming a user reflects on the next reload without a backfill job.
RequiresRestart reserves UI affordances for P3 (banner "this change won't take effect until the next restart"). In P1 the only seeded key is per-request, so always false.
ValueType is one of "int", "string", "bool". Service layer rejects updates whose payload type does not match; UI uses it to pick InputNumber vs Input vs Switch.