fix: read real users from eventify-django SQLite via secondary database

The admin_api was querying eventify-backend's empty PostgreSQL. Real users
live in eventify-django's SQLite (db.sqlite3 on host). Fix:

- settings.py: auto-adds 'users_db' database config when users_db.sqlite3
  is mounted into the container (read-only volume in docker-compose)
- views.py: _user_db() helper selects the correct database alias;
  _user_qs() defers 'partner' field (absent from older SQLite schema)
- UserMetricsView, UserListView, UserDetailView, UserStatusView all use
  _user_qs() so they query the 25 real registered customers

eventify/settings.py

@@ -176,3 +176,16 @@ SIMPLE_JWT = {
     'USER_ID_FIELD': 'id',
     'USER_ID_CLAIM': 'user_id',
 }
+
+# ---------------------------------------------------------------------------
+# Secondary read-only database: real user data from eventify-django SQLite
+# Activated automatically when users_db.sqlite3 is mounted into the container
+# ---------------------------------------------------------------------------
+import os as _os
+_sqlite_users = BASE_DIR / 'users_db.sqlite3'
+if _os.path.exists(_sqlite_users):
+    DATABASES['users_db'] = {
+        'ENGINE': 'django.db.backends.sqlite3',
+        'NAME': _sqlite_users,
+        'OPTIONS': {'check_same_thread': False},
+    }