Add expected checkout preview API using property billing policy defaults

docs/API_REFERENCE.txt

@@ -1655,6 +1655,53 @@ BOOKING APIS
   - 404 Not Found
 
 
+- Preview expected checkout API is this one:
+
+  POST /properties/{propertyId}/bookings/expected-checkout-preview
+
+  What it does:
+
+  - Computes expected check-out timestamp from check-in + billing policy.
+  - `checkInAt` is required.
+  - `billableNights` is optional (defaults to 1).
+  - `billingMode` is optional (defaults to property policy mode behavior).
+  - `billingCheckinTime` and `billingCheckoutTime` are optional; defaults come from property billing policy.
+  - `propertyId` is required in path.
+
+  Request body (BookingExpectedCheckoutPreviewRequest):
+
+  {
+    "checkInAt": "2026-02-05T10:30:00+05:30",
+    "billableNights": 2,
+    "billingMode": "PROPERTY_POLICY",
+    "billingCheckinTime": "12:00",
+    "billingCheckoutTime": "11:00"
+  }
+
+  Response body (BookingExpectedCheckoutPreviewResponse):
+
+  {
+    "expectedCheckOutAt": "2026-02-07T11:00+05:30",
+    "billableNights": 2,
+    "billingMode": "PROPERTY_POLICY",
+    "billingCheckinTime": "12:00",
+    "billingCheckoutTime": "11:00"
+  }
+
+  - Allowed roles: ADMIN, MANAGER, STAFF, HOUSEKEEPING, FINANCE
+
+  Error Codes
+
+  - 400 Bad Request
+      - checkInAt required or invalid timestamp
+      - billableNights must be > 0
+      - Unknown billing mode
+      - billingCheckinTime/billingCheckoutTime invalid format
+  - 401 Unauthorized
+  - 403 Forbidden
+  - 404 Not Found (property)
+
+
 - Booking stream API is this one:
 
   GET /properties/{propertyId}/bookings/{bookingId}/stream