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.

eventify/settings.py

@@ -176,16 +176,3 @@ 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},
-    }