Security: mask settings secrets, remove hardcoded employee-DB creds

- GET /settings now masks password/token values (were returned in plaintext
  to anonymous callers); sending the mask back on update is a no-op so the
  real secret is never clobbered.
- Move the employee-directory DB credentials out of source into env-backed
  config (shopdb.utils.employee_db); employees + notification recognition use
  the shared helper. Employee lookups stop leaking exception strings.
- Fix low-supplies report using loc.location instead of loc.locationname.

Employee/notification read endpoints stay unauthenticated by design (public
shopfloor kiosk displays consume them).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/printers/api/asset_routes.py

@@ -565,7 +565,7 @@ def _get_low_supplies_data():
                 from shopdb.core.models import Location
                 loc = Location.query.get(asset.locationid)
                 if loc:
-                    location_name = loc.location
+                    location_name = loc.locationname
 
             results.append({
                 'printerid': printer.printerid,