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_fields.html

@@ -1,7 +1,20 @@
 {{define "asset_fields_only"}}
-{{if .}}
+{{if not .IsUnique}}
+<div class="mb-3">
+  <label class="form-label" for="assetFormQuantity">Quantity</label>
+  <div class="input-group" style="max-width: 220px">
+    <button class="btn btn-outline-secondary" type="button" data-action="quantity-step" data-step="-1">&minus;</button>
+    <input type="number" class="form-control text-center" name="quantity" id="assetFormQuantity" min="0" step="1" value="{{.Quantity}}">
+    <button class="btn btn-outline-secondary" type="button" data-action="quantity-step" data-step="1">+</button>
+  </div>
+  <div class="form-text">This item type isn't Unique, so it's tracked by quantity instead of one row per unit.</div>
+</div>
+{{else}}
+<input type="hidden" name="quantity" value="1">
+{{end}}
+{{if .Fields}}
 <h6 class="mt-3 mb-2">Type-Specific Fields</h6>
-{{range .}}
+{{range .Fields}}
 {{$fv := .}}
 <div class="mb-2">
   <label class="form-label" for="field-{{.Def.ID}}">{{.Def.Name}}{{if .Def.Required}} <span class="text-danger">*</span>{{end}}</label>