fix: comprehensive PostgreSQL/SQLite compatibility fixes

Critical fixes for database compatibility issues:

INSERT operations:
- Fix all INSERT queries to use .returning('id')
- Handle both PostgreSQL (returns objects) and SQLite (returns IDs)
- Fixed in: events.js, adminArchives.js, adminEvents-enhanced.js, create-test-event.js

Date operations:
- Replace SQLite-specific db.raw("datetime('now', '+30 days')")
- Use JavaScript Date objects for cross-database compatibility
- Fixed in: adminArchives.js

Database utilities:
- Add dbCompat.js utility for handling database differences
- Provides consistent API for inserts, dates, booleans, and DB-specific operations
- Centralized database compatibility logic

Migration:
- Add migration 023 documenting PostgreSQL compatibility requirements
- Ensures future developers are aware of compatibility needs

This resolves all 'not iterable' errors and ensures the application
works correctly with both PostgreSQL (production) and SQLite (development).

backend/scripts/create-test-event.js

@@ -36,7 +36,8 @@ async function createTestEvent() {
     await db('events').where('slug', eventData.slug).delete();
     
     // Insert new event
-    const [eventId] = await db('events').insert(eventData);
+    const insertResult = await db('events').insert(eventData).returning('id');
+    const eventId = insertResult[0]?.id || insertResult[0];
     console.log('Event created with ID:', eventId);
     
     console.log('\nTest event created successfully!');