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.

internal/assets/csv.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"sort"
+	"strconv"
 	"strings"
 
 	"inventree/internal/models"
@@ -19,6 +20,7 @@ const (
 	colStatus       = "status"
 	colAssignedUser = "assigned user"
 	colNotes        = "notes"
+	colQuantity     = "quantity"
 	cfColPrefix     = "cf_"
 )
 
@@ -201,6 +203,13 @@ func (s *Store) Import(ctx context.Context, r io.Reader, opts ImportOptions,
 			}
 		}
 
+		quantity := 1
+		if qtyStr := get(row, colQuantity); qtyStr != "" {
+			if n, err := strconv.Atoi(qtyStr); err == nil {
+				quantity = n
+			}
+		}
+
 		asset := &models.Asset{
 			AssetTag:       get(row, colAssetID),
 			Name:           name,
@@ -209,6 +218,7 @@ func (s *Store) Import(ctx context.Context, r io.Reader, opts ImportOptions,
 			StatusID:       statusID,
 			AssignedUserID: assignedUserID,
 			Notes:          get(row, colNotes),
+			Quantity:       quantity,
 		}
 
 		if asset.AssetTag != "" {
@@ -282,7 +292,7 @@ func (s *Store) Export(ctx context.Context, w io.Writer, f ListFilter) error {
 	}
 
 	cw := csv.NewWriter(w)
-	header := []string{"Asset ID", "Name", "Item Type", "Folder", "Status", "Assigned User", "Notes", "Created At", "Updated At"}
+	header := []string{"Asset ID", "Name", "Item Type", "Folder", "Status", "Assigned User", "Notes", "Quantity", "Created At", "Updated At"}
 	for _, k := range fieldDefs {
 		header = append(header, cfColPrefix+k)
 	}
@@ -304,6 +314,7 @@ func (s *Store) Export(ctx context.Context, w io.Writer, f ListFilter) error {
 			}
 			row := []string{
 				a.AssetTag, a.Name, a.ItemTypeName, folderPathString(a.FolderPath), a.StatusName, a.AssignedUsername, a.Notes,
+				strconv.Itoa(a.Quantity),
 				a.CreatedAt.Format("2006-01-02 15:04:05"), a.UpdatedAt.Format("2006-01-02 15:04:05"),
 			}
 			values := map[string]string{}