Stop the till and Nearle Daily from sharing accounts

app_users is the only thing the two products have in common, and the code was
treating it as though it were the whole relationship. Both directions leaked.

Back-office roles were leaking into the till. PosRoleCanManageStaff returned
true for roleid 1 to 6, on the reasoning that somebody who already administers a
shop from a browser is not made less privileged by standing at the counter. That
sounds fine and is wrong: measured against live data it handed till-supervisor
powers to 68 accounts, 59 of them Nearle Daily Super admins, not one of whom is
the administrator of anybody's POS. Meanwhile the actual shop accounts carry
roleid 0 and were refused, so the mapping was backwards from intent in both
halves at once.

Till accounts were leaking into the application. GetStaffs is WHERE tenantid
with no role filter, so a Counter Cashier appeared in the tenant staff list
beside the delivery riders — a row every action on that page would fail against,
since a cashier has no app login, no rider shift and no back-office screen.

So: eligibility for a till is now granted explicitly by provisioning a
Supervisor or a Cashier, never inherited from a back-office role, and roles 7
and 8 are excluded from every Nearle Daily lookup. The exclusion lives in the
queries rather than in a check after them, because a check bolted on afterwards
has to be repeated at six call sites and is one edit away from being forgotten
at one of them — and that one would be the hole. A till account is not rejected
by the app login; it is not found.

Two things this surfaced that were not visible before.

A Supervisor could not open a till. PIN sign-in needs a session that already
exists, so once back-office roles were refused, an outlet whose only POS
accounts were PIN-only had no way in at all. Supervisors are now provisioned
with a username and password as well as a PIN; cashiers deliberately get neither,
because they sign on at a counter somebody has already opened and a second
password would be one more credential to leak for no capability gained.

UpdatePosUser silently dropped authname. It wrote the password, reported
success, and left the account unreachable by either lookup — the failure
surfaced at a counter as "not recognised" rather than on the screen that caused
it. Contactno had the same gap.

Verified against live rows rather than asserted, by scratch/posseparation: a
provisioned supervisor signs in and gets the supervisor shell; five real
back-office accounts including Super admins are refused; the supervisor is
invisible to applogin, tenant weblogin and the password-setup lookup; and no
till account appears in getallusers, while asking for role 7 by name still
returns them so the console can read its own people.

All five outlets that stock products now have a Supervisor and a Cashier.

Also moves the loose markdown into docs/, which was already staged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

repositories/posAuthRepository.go

@@ -101,6 +101,18 @@ func (r *posRepository) PosLogin(req models.PosLoginRequest) (*models.PosSession
 // reaches more than one outlet. It is checked against that set, never trusted
 // on its own.
 func (r *posRepository) sessionFor(row posLoginRow, requestedLocation int) (*models.PosSession, error) {
+	// The till is not the back office, and one account is never both. An
+	// account reaches a terminal only by having been provisioned for one —
+	// Supervisor or Cashier, created from the console — and never by carrying a
+	// Nearle Daily role that happens to sound senior.
+	//
+	// Checked here rather than in PosLogin so that the PIN route is covered by
+	// the same line. Both ways in build their session through this function, and
+	// a gate on only one of them would be a gate on neither.
+	if !models.PosRoleEligible(row.Roleid) {
+		return nil, errPosRoleIneligible
+	}
+
 	if row.Tenantid <= 0 {
 		return nil, fmt.Errorf("this account is not attached to a tenant and cannot open a till")
 	}
@@ -324,6 +336,17 @@ func (r *posRepository) PosLocationAllowed(tenantID, locationID int) (bool, erro
 // errPosLoginRejected is the single answer to a bad email and a bad password.
 var errPosLoginRejected = fmt.Errorf("those sign-in details were not recognised")
 
+// errPosRoleIneligible is the answer to a correct credential on an account that
+// is not a till account.
+//
+// Deliberately specific, where a bad password is deliberately vague. By the
+// time this fires the caller has already proved the credential, so naming the
+// reason leaks nothing they did not just demonstrate — and the vague answer
+// would send a shop owner hunting for a password that was never wrong. It
+// names the fix, because the fix is somebody else's screen.
+var errPosRoleIneligible = fmt.Errorf(
+	"this account is not set up for the till; ask your store admin to add you as a Supervisor or Cashier in the web console")
+
 // PosLoginRejected reports whether an error is a failed credential check, so
 // the controller can answer 401 for those and 500 for a database fault without
 // matching on message text.