feat(19-04): add users.handle, HandleGenerator and user.Created

Generate a unique mention handle on API register and admin create, and fire user.Created from both paths so listeners run in the same write.

Co-authored-by: Cursor <cursoragent@cursor.com>

models/user.go

@@ -33,6 +33,10 @@ type User struct {
 	TokensValidAfter          *time.Time `gorm:"column:tokens_valid_after"`
 	CreatedIPAddress          *string    `gorm:"column:created_ip_address"`
 	LastIPAddress             *string    `gorm:"column:last_ip_address"`
+	// Handle is the unique @mention identity (D-15). Generated on create,
+	// never mass-assigned, and omitted from user API payloads unless a
+	// golden requires it.
+	Handle *string `gorm:"column:handle" json:"-"`
 	// LastSeen is stamped by login and token refresh, at most once per five
 	// minutes (classes.TouchLastSeen). It is shown in the admin list and is
 	// never part of a user API payload.