Give every terminal its own identity, and report fleet presence

Answers "which of my 100 tills are alive and healthy", and fixes three things
that were fine on one device and broken on a hundred.

Terminal identity (lib/data/local/terminal_identity.dart)
- Every device mints a UUID on first run, stored in its own database, plus a
  short code (T4A9) derived from it. Renaming keeps the device id, so history
  keeps pointing at the same physical till.
- Replaces the literal 'TERM-01', which was hardcoded in five places. The whole
  fleet reported as one terminal: shift reports merged, MQTT topics collided,
  and a second connection with the same client id evicts the first from the
  broker — so two tills would have knocked each other offline in a loop.

Invoice numbers now carry the terminal code
- INV-2608-T4A9-00042. The sequence counter lives in each till's own database
  and starts at 1, so without this every terminal in the fleet minted
  INV-2608-00001 for its first sale of the month. The order UUID kept the data
  distinct; the number a customer quotes on a receipt was not.

SQLite pragmas
- WAL, so the product grid refreshing does not block the sale being written,
  and the file is never left mid-rewrite by a power cut.
- busy_timeout 5s, so a contended lock waits instead of throwing "database is
  locked" — which at checkout is a failed sale with a customer standing there.
- synchronous NORMAL, the right trade under WAL for a till.

Fleet presence (lib/data/sync/presence_reporter.dart)
- Retained status record on connect and once a minute: device id, code, name,
  app version, pending bill count, last upload, catalogue revision, sync halt
  state. Retained so a dashboard connecting at noon gets all 100 terminals
  immediately rather than a blank board.
- The Last Will already said "reachable". A till can be connected and still be
  holding 200 unsent bills or running last month's prices; only pending_bills
  and catalogue_revision say so.

NATS
- The MQTT gateway maps / to . so the existing transport works unchanged.
  SyncConfig.asNatsSubject() exposes the translation, and the contract doc
  gives the JetStream subjects (pos.*.*.order, pos.*.*.status) plus the two
  server-side requirements: a file-backed stream, and the ack published by the
  consumer after commit rather than by the ingest handler.

Tests: 129 -> 140. New coverage for identity minting and stability, per-device
invoice uniqueness, topic and client-id separation, NATS subject mapping, and
the two pragmas. Suite run three times clean.

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

lib/core/config/sync_config.dart

@@ -10,6 +10,11 @@ enum TransportKind {
   /// Persistent connection. Costs a broker, and buys the downlink: head office
   /// can push a price change or ask a terminal to sync without waiting for it
   /// to ask first.
+  ///
+  /// Speaks MQTT 3.1.1, so it works against Mosquitto, EMQX or a NATS server
+  /// with its MQTT gateway enabled. Against NATS the topics below arrive as
+  /// subjects with `/` mapped to `.` — `pos/store-01/T4A9/order` becomes
+  /// `pos.store-01.T4A9.order` — which is what a JetStream consumer binds to.
   mqtt,
 }
 
@@ -90,8 +95,20 @@ class SyncConfig {
 
   /// Stable across restarts so the broker can resume a session and redeliver
   /// anything in flight, rather than treating each launch as a new client.
+  ///
+  /// Derived from the terminal's own identity, which is minted per device — two
+  /// tills sharing a client id would knock each other off the broker in a loop,
+  /// since a second connection with the same id evicts the first.
   String get clientId => 'pos-$storeId-$terminalId';
 
+  /// The same topic as a NATS subject.
+  ///
+  /// NATS' MQTT gateway maps `/` to `.`, so this is what a JetStream stream or
+  /// consumer is configured against. Provided so the wildcard a back-office
+  /// consumer needs can be read off the terminal rather than guessed:
+  /// `pos.*.*.order` for every till's bills, `pos.*.*.status` for presence.
+  static String asNatsSubject(String topic) => topic.replaceAll('/', '.');
+
   SyncConfig copyWith({
     TransportKind? transport,
     String? storeId,