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/panel_asset.html

@@ -25,7 +25,19 @@
         <div class="card-header py-2 fw-semibold">Details</div>
         <table class="table table-sm mb-0">
           <tbody>
-            <tr><th class="w-25">Assigned User</th><td>{{if .Asset.AssignedUsername}}{{.Asset.AssignedUsername}}{{else}}<span class="text-body-secondary">Unassigned</span>{{end}}</td></tr>
+            <tr><th class="w-25">Assigned User</th><td>{{if .Asset.AssignedUsername}}{{.Asset.AssignedUsername}}{{else}}<span class="text-body-secondary">Unassigned &mdash; right-click above to check out</span>{{end}}</td></tr>
+            {{if not .Asset.ItemTypeIsUnique}}
+            <tr>
+              <th>Quantity</th>
+              <td>
+                <div class="input-group input-group-sm" style="max-width: 150px" data-quantity-stepper data-asset-id="{{.Asset.ID}}">
+                  <button class="btn btn-outline-secondary" type="button" data-action="panel-quantity-step" data-step="-1">&minus;</button>
+                  <span class="form-control text-center" data-quantity-value>{{.Asset.Quantity}}</span>
+                  <button class="btn btn-outline-secondary" type="button" data-action="panel-quantity-step" data-step="1">+</button>
+                </div>
+              </td>
+            </tr>
+            {{end}}
             <tr><th>Created</th><td>{{fmtDate .Asset.CreatedAt}}</td></tr>
             <tr><th>Updated</th><td>{{fmtDate .Asset.UpdatedAt}}</td></tr>
           </tbody>