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_folder.html

@@ -38,13 +38,23 @@
   {{if .Assets}}
   <div class="table-responsive">
     <table class="table table-sm table-hover asset-table">
-      <thead><tr><th>Asset ID</th><th>Name</th><th>Type</th><th>Status</th><th>Assigned</th></tr></thead>
+      <thead><tr><th>Asset ID</th><th>Name</th><th>Type</th><th>Qty</th><th>Status</th><th>Assigned</th></tr></thead>
       <tbody>
         {{range .Assets}}
         <tr class="asset-row" data-id="{{.ID}}">
           <td>{{.AssetTag}}</td>
           <td><i class="bi {{iconOr .ItemTypeIcon "bi-box-seam"}} me-1"></i>{{.Name}}</td>
           <td>{{.ItemTypeName}}</td>
+          <td>
+            {{if .ItemTypeIsUnique}}<span class="text-body-secondary">&mdash;</span>
+            {{else}}
+            <div class="input-group input-group-sm" style="width:6.5rem" data-quantity-stepper data-asset-id="{{.ID}}">
+              <button class="btn btn-outline-secondary py-0" type="button" data-action="table-quantity-step" data-step="-1">&minus;</button>
+              <span class="form-control text-center py-0" data-quantity-value>{{.Quantity}}</span>
+              <button class="btn btn-outline-secondary py-0" type="button" data-action="table-quantity-step" data-step="1">+</button>
+            </div>
+            {{end}}
+          </td>
           <td><span class="badge text-bg-{{.StatusColor}}">{{.StatusName}}</span></td>
           <td>{{if .AssignedUsername}}{{.AssignedUsername}}{{else}}<span class="text-body-secondary">&mdash;</span>{{end}}</td>
         </tr>