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/pages/labels.html

@@ -0,0 +1,50 @@
+{{define "content"}}
+<style>{{.SheetCSS}}
+@media print {
+  .app-navbar, .print-toolbar { display: none !important; }
+  body { margin: 0; background: #fff; }
+  .label-sheet { margin: 0; }
+}
+.label-tag { font-family: ui-monospace, SFMono-Regular, Menlo, monospace; font-weight: 700; font-size: 11pt; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
+.label-name { font-size: 8pt; color: #333; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+</style>
+
+<div class="print-toolbar d-flex flex-wrap align-items-center gap-3 p-3 border-bottom">
+  <button class="btn btn-primary" onclick="window.print()"><i class="bi bi-printer"></i> Print</button>
+  <span class="text-body-secondary">{{len .Labels}} label(s)</span>
+  <form method="get" class="d-flex align-items-center gap-2 ms-auto">
+    {{if .AssetID}}<input type="hidden" name="asset_id" value="{{.AssetID}}">{{end}}
+    {{if .FolderID}}<input type="hidden" name="folder_id" value="{{.FolderID}}">{{end}}
+    {{if .Recursive}}<input type="hidden" name="recursive" value="1">{{end}}
+    <label class="form-label mb-0 small text-body-secondary" for="labelTemplateSelect">Label sheet</label>
+    <select class="form-select form-select-sm" id="labelTemplateSelect" name="template" onchange="this.form.submit()">
+      {{range $key := .TemplateKeys}}
+      {{$t := index $.Templates $key}}
+      <option value="{{$key}}" {{if eq $key $.Selected.Key}}selected{{end}}>{{$t.DisplayName}}</option>
+      {{end}}
+    </select>
+  </form>
+</div>
+
+{{if .Selected.Approximate}}
+<div class="alert alert-warning m-3">
+  This layout's grid/margins aren't from a verified spec sheet - print one test page on plain paper and
+  hold it up to a real label sheet before printing a full (often waterproof, non-refundable) sheet.
+</div>
+{{end}}
+
+<div class="label-sheet">
+  {{range .Labels}}
+  <div class="label">
+    <img src="{{.QRData}}" alt="QR code for {{.Asset.AssetTag}}">
+    <div class="min-w-0">
+      <div class="label-tag">{{.Asset.AssetTag}}</div>
+      <div class="label-name">{{.Asset.Name}}</div>
+    </div>
+  </div>
+  {{end}}
+</div>
+{{if not .Labels}}
+<p class="text-body-secondary p-3">No assets to print.</p>
+{{end}}
+{{end}}