Add registration_activity chart and enhance admin statistics

- Introduced `RegistrationActivityChart` to display user registration trends over 14 days.
- Enhanced `AdminStatsResponse` with `registration_activity`, providing improved insights for admin users.
- Updated demo data to include realistic registration activity and organization details.
- Refactored admin page to use updated statistics data model and improved query handling.
- Fixed inconsistent timezone handling in statistical analytics and demo user timestamps.

backend/app/init_db.py

@@ -9,7 +9,7 @@ import asyncio
 import json
 import logging
 import random
-from datetime import datetime, timedelta
+from datetime import UTC, datetime, timedelta
 from pathlib import Path
 
 from sqlalchemy import select, text
@@ -153,23 +153,24 @@ async def load_demo_data(session):
                 # Randomize created_at for demo data (last 30 days)
                 # This makes the charts look more realistic
                 days_ago = random.randint(0, 30)  # noqa: S311
-                random_time = datetime.utcnow() - timedelta(days=days_ago)
+                random_time = datetime.now(UTC) - timedelta(days=days_ago)
                 # Add some random hours/minutes variation
                 random_time = random_time.replace(
                     hour=random.randint(0, 23),  # noqa: S311
                     minute=random.randint(0, 59),  # noqa: S311
                 )
 
-                # Update the timestamp directly in the database
+                # Update the timestamp and is_active directly in the database
+                # We do this to ensure the values are persisted correctly
                 await session.execute(
                     text(
-                        "UPDATE users SET created_at = :created_at WHERE id = :user_id"
+                        "UPDATE users SET created_at = :created_at, is_active = :is_active WHERE id = :user_id"
                     ),
-                    {"created_at": random_time, "user_id": user.id},
+                    {"created_at": random_time, "is_active": user_data.get("is_active", True), "user_id": user.id},
                 )
 
                 logger.info(
-                    f"Created demo user: {user.email} (created {days_ago} days ago)"
+                    f"Created demo user: {user.email} (created {days_ago} days ago, active={user_data.get('is_active', True)})"
                 )
 
                 # Add to organization if specified