refactor: migrate users to PostgreSQL, remove SQLite secondary DB

Users have been migrated from eventify-django SQLite to eventify-backend
PostgreSQL. The temporary users_db workaround is no longer needed:

- settings.py: removed users_db SQLite secondary database config
- views.py: removed _user_db()/_user_qs() helpers; user views now query
  the default PostgreSQL directly with plain User.objects.filter()
- docker-compose.yml: SQLite read-only volume mount removed

All 27 users (25 non-superuser customers) now live in PostgreSQL.

admin_api/views.py

@@ -488,19 +488,6 @@ _USER_ROLE_MAP = {
     'partner_staff': 'Partner', 'partner_customer': 'Partner',
 }
 
-def _user_db():
-    """Return the database alias that holds real user data.
-    Uses 'users_db' (SQLite from eventify-django) when mounted, else 'default'."""
-    from django.conf import settings
-    return 'users_db' if 'users_db' in settings.DATABASES else 'default'
-
-
-def _user_qs(User):
-    """Base queryset for end-user lookups: correct database, partner field deferred
-    (SQLite schema predates the partner_id column added in eventify-backend)."""
-    return User.objects.using(_user_db()).defer('partner')
-
-
 def _serialize_user(u):
     full_name = f'{u.first_name} {u.last_name}'.strip() or u.username
     role_key = u.role if u.role else ('customer' if getattr(u, 'is_customer', False) else 'staff')
@@ -553,7 +540,7 @@ class UserMetricsView(APIView):
         today = timezone.now().date()
         week_ago = today - datetime.timedelta(days=7)
         # Customers = all non-superuser accounts (end users registered via mobile/web)
-        customer_qs = _user_qs(User).filter(is_superuser=False)
+        customer_qs = User.objects.filter(is_superuser=False)
         return Response({
             'total':       customer_qs.count(),
             'active':      customer_qs.filter(is_active=True).count(),
@@ -570,7 +557,7 @@ class UserListView(APIView):
         from django.db.models import Q
         User = get_user_model()
         # Customers = all non-superuser accounts (end users registered via mobile/web)
-        qs = _user_qs(User).filter(is_superuser=False)
+        qs = User.objects.filter(is_superuser=False)
         if s := request.GET.get('status'):
             if s == 'Active':
                 qs = qs.filter(is_active=True)
@@ -604,7 +591,7 @@ class UserDetailView(APIView):
         from django.contrib.auth import get_user_model
         from django.shortcuts import get_object_or_404
         User = get_user_model()
-        u = get_object_or_404(_user_qs(User), pk=pk)
+        u = get_object_or_404(User, pk=pk)
         return Response(_serialize_user(u))
 
 
@@ -615,7 +602,7 @@ class UserStatusView(APIView):
         from django.contrib.auth import get_user_model
         from django.shortcuts import get_object_or_404
         User = get_user_model()
-        u = get_object_or_404(_user_qs(User), pk=pk)
+        u = get_object_or_404(User, pk=pk)
         action = request.data.get('action')
         if action in ('suspend', 'ban'):
             u.is_active = False