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/asset_form.html

@@ -49,7 +49,7 @@
       <select class="form-select" name="assigned_user_id">
         <option value="">Unassigned</option>
         {{range .Users}}
-        <option value="{{.ID}}" {{if eq $selectedUser .ID}}selected{{end}}>{{.Username}}</option>
+        <option value="{{.ID}}" {{if eq $selectedUser .ID}}selected{{end}}>{{.Username}}{{if eq .Role "contact"}} (Contact){{end}}</option>
         {{end}}
       </select>
     </div>
@@ -60,7 +60,7 @@
     </div>
 
     <div class="col-12" id="asset-fields-container">
-      {{template "asset_fields_only" .Fields}}
+      {{template "asset_fields_only" (dict "IsUnique" .SelectedITUnique "Quantity" .Quantity "Fields" .Fields)}}
     </div>
   </div>