Upload shopper registrations, and charge GST to the lines that earned it

Two defects that share a shape: a figure landing on the wrong record.

Bill-level discounts were apportioned across every line by a single
factor, so "20% off Beverages" pulled tax out of the atta line as well.
The bill total was right either way, which is what made it easy to ship
— only the slab split on a filed return was wrong. Targeted campaigns
now reduce the lines they name, and bill-wide reductions still spread
pro rata, so the arithmetic is unchanged wherever it was already right.

Shoppers registered at a till only ever reached the back office as three
fields riding along on a bill. Somebody who signed up and bought nothing
existed on one terminal and nowhere else, and two tills registering the
same mobile each minted their own row. Customers are now an outbox of
their own on pos/{store}/{terminal}/customer, and the id is a UUIDv5
over the normalised mobile number — so a hundred terminals agree on who
a shopper is without talking to each other.

Registrations go up before bills, and a failure there cannot strand a
day's takings. No loyalty figures are sent: they belong to the bill
stream, which is idempotent and knows about every counter.

Two things found while building it. Numbers were keyed on raw digits, so
a cashier typing +91 forked a shopper as effectively as a random id
would. And the sale path wrote the customer with ConflictAlgorithm
.replace, which is a DELETE and an INSERT — every column absent from the
row reverts to its schema default, so the new sync flag would have been
cleared by the shopper's next purchase.

Schema v8. Existing customers are queued rather than assumed sent: the
terminal cannot tell an imported row from a locally registered one, and
only one of those mistakes loses somebody.

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

lib/domain/entities/customer.dart

@@ -1,4 +1,5 @@
 import 'package:equatable/equatable.dart';
+import 'package:uuid/uuid.dart';
 
 import '../../core/constants/app_constants.dart';
 import '../../core/utils/extensions.dart';
@@ -74,6 +75,51 @@ class Customer extends Equatable {
   final DateTime? createdAt;
   final DateTime? lastVisitAt;
 
+  /// Fixed namespace for customer ids. Must never change: it is half the
+  /// input to [idForMobile], so a new one renames every shopper in the fleet.
+  static const _namespace = '9f2b7c14-3d6e-5a80-b1f7-2c4e8a05d913';
+
+  static const _uuid = Uuid();
+
+  /// The id for the shopper reachable on [mobile].
+  ///
+  /// Derived from the number rather than minted at random, which is what lets
+  /// a hundred terminals agree without talking to each other. A shopper who
+  /// registers at counter 2 in Anna Nagar and shops at counter 5 in T Nagar
+  /// gets the same id both times, so the back office collapses them on a
+  /// primary key instead of guessing at a merge later.
+  static String idForMobile(String mobile) =>
+      _uuid.v5(_namespace, normaliseMobile(mobile));
+
+  /// Every digit in [mobile], in order. Used for matching what a cashier types
+  /// against what is stored, where a partial number should still find a row.
+  static String digitsOf(String mobile) => mobile.replaceAll(RegExp(r'\D'), '');
+
+  /// Reduces a number to the ten-digit national one identity is keyed on.
+  ///
+  /// One cashier types `+91 98400 12345`, another `098400 12345`, a third
+  /// `9840012345`. Keyed on raw digits those are three different shoppers,
+  /// which is precisely the duplication [idForMobile] exists to prevent — the
+  /// country code would fork a customer just as effectively as a random id.
+  ///
+  /// Only the two prefixes an Indian number actually carries are stripped, and
+  /// only at the exact lengths that make them unambiguous. Anything else is
+  /// left alone: mangling a number this rule was not written for is worse than
+  /// storing it verbatim.
+  static String normaliseMobile(String mobile) {
+    final digits = digitsOf(mobile);
+
+    // +91 98400 12345
+    if (digits.length == 12 && digits.startsWith('91')) {
+      return digits.substring(2);
+    }
+    // 0 98400 12345 — the old STD trunk prefix, still muscle memory for many.
+    if (digits.length == 11 && digits.startsWith('0')) {
+      return digits.substring(1);
+    }
+    return digits;
+  }
+
   MembershipTier get tier => MembershipTier.forSpend(lifetimeSpend);
 
   /// Cash value of the points currently held.