Add asset checkout, quantity/uniqueness, contacts, and QR labels

Checkout:
- assets.AssignUser + POST /app/assets/{id}/checkout assign/unassign an
  asset's user independent of the full edit form. New shared
  #checkoutModal, wired from a "Check Out..." context-menu action, backed
  by GET /app/assets/{id} and GET /app/users/options JSON endpoints.

Quantity / uniqueness:
- Item types gain an IsUnique flag (checked by default, matching prior
  behavior). Assets gain a Quantity column, fixed at 1 for unique types
  and adjustable via +/- steppers (POST /app/assets/{id}/quantity) for
  non-unique ones. Flipping a type back to unique clamps existing assets'
  quantity back to 1. CSV import/export gained a Quantity column.
  Migration 0003_quantity.sql.

Contacts:
- New models.RoleContact: an assignable person who cannot sign in
  (User.CanLogin). Migration 0004_contacts.sql rebuilds the users table
  (SQLite can't alter a CHECK constraint in place) to allow the role.
  Users form/list updated; contact creation auto-generates an unusable
  password since the login path is blocked by role regardless.

QR asset tags and printable labels:
- Each asset's QR code (via github.com/skip2/go-qrcode, pure Go) encodes
  an absolute /scan/<tag> URL; GET /scan/{tag} resolves it and redirects
  into /inventory?asset=<id>, which app.js picks up on load to
  auto-select that asset - works from a phone camera or a USB
  scanner-as-keyboard-wedge pointed at the address bar.
- GET /labels renders a print-ready sheet (right-click an asset for one
  label, a folder for every asset in it) with a template switcher between
  Avery 5160 (verified dimensions) and 61526 asset tag labels (best-effort
  layout, flagged as such on the page).

Also refreshed README to reflect the right-click-first tree UX and all of
the above.

web/templates/partials/modals.html

@@ -128,6 +128,29 @@
   </div>
 </div>
 
+<!-- Check out / check in asset -->
+<div class="modal fade" id="checkoutModal" tabindex="-1">
+  <div class="modal-dialog">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="checkoutModalTitle">Check Out Asset</h5>
+        <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+      </div>
+      <div class="modal-body">
+        <p class="text-body-secondary small mb-3" id="checkoutCurrent"></p>
+        <label class="form-label">Assigned User</label>
+        <select class="form-select" id="checkoutUserSelect">
+          <option value="">Unassigned (check in)</option>
+        </select>
+      </div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-outline-secondary" data-bs-dismiss="modal">Cancel</button>
+        <button type="button" class="btn btn-primary" id="confirmCheckoutBtn">Save</button>
+      </div>
+    </div>
+  </div>
+</div>
+
 <!-- Import CSV into a specific folder (right-click a folder > Import CSV...) -->
 <div class="modal fade" id="importModal" tabindex="-1">
   <div class="modal-dialog">