server: exit flow (pay-on-foot validation)

A credential read at an exit lane validates the session, then opens. Adds a
'read' channel to the device bus (DeviceReadEvent: ticket/plate/qr/card);
entry stays button-driven so reads are exit/identity events.

Flow: read -> fold the SIGNED ledger for that identity -> validate open + PAID
+ within gracePeriodExitMin -> signed vehicle_exit -> pulseOpen -> close the
session cache. Unpaid / grace-expired / unknown -> signed anomaly, barrier
stays closed (a deliberate business reject, not a fail-state; 'exit fails open'
is about host/power loss). Validation reads the ledger (authoritative), not the
cache.

No payment events exist until the pay station is built, so every transient exit
currently rejects -- the correct end-state, not yet passable. Verified against
stubs: unpaid->anomaly+no-open; paid+grace->exit+open+closed; expired->anomaly;
unknown->anomaly; verifyChain ok across entry->pay->exit.

Flagged: lane_devices has no entry/exit direction model (exit door hardcoded to
1); needs a lane-direction/role model before multi-reader lanes.

apps/server/src/device-events.ts

@@ -15,6 +15,17 @@ export interface DeviceInputEvent {
   readonly source: "push" | "poll";
 }
 
+// A credential read at a lane: a ticket scanned at exit, a plate from LPR, a card
+// at a reader. Drives identity-based flows (exit validation, and later permits /
+// pay-station lookup). `kind` mirrors IdentitySource. See parking-session.md.
+export interface DeviceReadEvent {
+  readonly driverId: string;
+  readonly deviceId: string; // lane_devices id of the reader/scanner/camera
+  readonly value: string; // the ticket id / plate / card number
+  readonly kind: "ticket" | "plate" | "qr" | "card";
+  readonly at: string; // ISO-8601
+}
+
 /** A printer's status as tracked by the live monitor (status + identity). */
 export interface PrinterStatusEvent {
   readonly deviceId: string; // lane_devices id
@@ -33,6 +44,15 @@ class DeviceEventBus extends EventEmitter {
     return () => this.off("input", cb);
   }
 
+  /** A credential read (ticket scan, plate, card) at a lane. */
+  emitRead(event: DeviceReadEvent): void {
+    this.emit("read", event);
+  }
+  onRead(cb: (event: DeviceReadEvent) => void): () => void {
+    this.on("read", cb);
+    return () => this.off("read", cb);
+  }
+
   /** Emitted by the printer monitor whenever a printer's status CHANGES. */
   emitPrinterStatus(event: PrinterStatusEvent): void {
     this.emit("printer-status", event);