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>

docs/integration-guide.md

@@ -49,7 +49,7 @@ mqtt {
 authorization {
   users = [
     { user: "till", password: "…", permissions: {
-        publish:   ["pos.*.*.order", "pos.*.*.status"]
+        publish:   ["pos.*.*.order", "pos.*.*.customer", "pos.*.*.status"]
         subscribe: ["pos.*.*.ack", "pos.*.*.command", "pos.*.catalogue"]
     }}
   ]
@@ -193,6 +193,38 @@ for msg in subscribe("pos.*.*.order"):
 `ON CONFLICT DO NOTHING` still counts as accepted — a redelivery of a bill you
 already hold is a success, not a rejection.
 
+### Shopper registrations
+
+A second uplink runs on `pos/{store}/{terminal}/customer`, acked on the same
+topic by the same rules. Wire it the same way:
+
+```sql
+CREATE TABLE customers (
+  id          UUID PRIMARY KEY,        -- derived from the mobile number
+  mobile      TEXT NOT NULL UNIQUE,
+  name        TEXT NOT NULL,
+  email       TEXT,
+  gender      TEXT,
+  date_of_birth DATE,
+  registered_at TIMESTAMPTZ,
+  registered_by_terminal TEXT
+);
+```
+
+Insert with `ON CONFLICT (id) DO NOTHING` — **never** an upsert. A registration
+is replayed freely and must not overwrite a profile corrected at head office.
+
+The id is a UUIDv5 over the shopper's normalised ten-digit mobile, so two tills
+registering the same person independently produce the same row. Don't reassign
+it. And don't expect loyalty points in this payload: derive those from the bill
+stream, which is idempotent and knows about every counter.
+
+```bash
+nats sub 'pos.*.*.customer'
+```
+
+Add a shopper on the terminal — no sale needed — and it should appear.
+
 **Use `rejected` sparingly.** Naming an id there halts the terminal's drain: it
 stops retrying and waits for a person to press Sync. That is right for "this bill
 is malformed" and wrong for "my database is having a bad minute" — for the
@@ -264,7 +296,9 @@ and per-bill state — start there before the broker logs.
 ## Before a fleet rollout
 
 - **Back up `nearle_pos.db` on any terminal already trading.** The schema goes to
-  v7 on first launch and the migration is one-way.
+  v8 on first launch and the migration is one-way. It also queues every shopper
+  already on the terminal for upload, so expect one burst of registrations from
+  each existing store — collapse those onto the mobile number.
 - **Build per-ABI.** `flutter build apk --split-per-abi` gives ~23MB per
   architecture instead of a 69MB universal APK — worth it over shop wifi.
 - **Change the seed PINs.** `4821` / `5093` / `6274` are in the source. Every