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/settings_import.html

@@ -41,17 +41,19 @@
             Required: <code>Name</code>, <code>Item Type</code>, <code>Status</code>. Optional:
             <code>Asset ID</code> (auto-generated if blank; matches an existing asset to update it),
             <code>Folder</code> (a <code>/</code>-separated path such as <code>Main Office/Network</code>),
-            <code>Assigned User</code>, <code>Notes</code>, and one <code>cf_&lt;field_key&gt;</code> column
-            per custom field (e.g. <code>cf_serial_number</code>).
+            <code>Assigned User</code>, <code>Notes</code>, <code>Quantity</code> (defaults to 1; only
+            meaningful for item types that aren't marked Unique), and one <code>cf_&lt;field_key&gt;</code>
+            column per custom field (e.g. <code>cf_serial_number</code>).
           </p>
           <div class="table-responsive">
             <table class="table table-sm table-bordered mb-0 small">
               <thead>
-                <tr><th>Asset ID</th><th>Name</th><th>Item Type</th><th>Folder</th><th>Status</th><th>Assigned User</th><th>Notes</th><th>cf_serial_number</th></tr>
+                <tr><th>Asset ID</th><th>Name</th><th>Item Type</th><th>Folder</th><th>Status</th><th>Assigned User</th><th>Notes</th><th>Quantity</th><th>cf_serial_number</th></tr>
               </thead>
               <tbody>
-                <tr><td></td><td>Core Switch</td><td>Switch</td><td>Main Office/Network</td><td>In Use</td><td></td><td></td><td>SN-12345</td></tr>
-                <tr><td>IT-000042</td><td>Front Desk PC</td><td>Desktop</td><td>Main Office/Workstations</td><td>In Use</td><td>jdoe</td><td>Loaner unit</td><td></td></tr>
+                <tr><td></td><td>Core Switch</td><td>Switch</td><td>Main Office/Network</td><td>In Use</td><td></td><td></td><td></td><td>SN-12345</td></tr>
+                <tr><td>IT-000042</td><td>Front Desk PC</td><td>Desktop</td><td>Main Office/Workstations</td><td>In Use</td><td>jdoe</td><td>Loaner unit</td><td></td><td></td></tr>
+                <tr><td></td><td>USB-A Cable</td><td>Cable</td><td>Main Office/Spares</td><td>Spare</td><td></td><td></td><td>25</td><td></td></tr>
               </tbody>
             </table>
           </div>