feat: add optional event date and expiration settings

Add global settings to make event_date and expiration optional when
creating galleries. This supports non-event use cases like portraits,
corporate shoots, etc.

New features:
- Settings toggles in Settings → Event Creation tab
- "Require event date" checkbox with warning about random URL identifiers
- "Require expiration date" checkbox with warning about manual archiving
- Galleries without date use random hex suffix in slug (e.g. portrait-smith-a1b2c3)
- Galleries without expiration never expire (stay active until archived)

Backend changes:
- New migration for settings and nullable columns
- Conditional validation based on settings
- Updated slug generation with random suffix fallback
- Updated expiration checker to skip null expires_at
- Updated gallery access control for null expiration

Frontend changes:
- New checkboxes in EventsTab with warnings
- Conditional event date field (shows optional label)
- No Expiration message when expiration disabled
- Updated types for nullable event_date and expires_at

Closes #118

backend/src/services/expirationChecker.js

@@ -21,9 +21,11 @@ async function checkExpirations() {
     const warningDate = new Date(now.getTime() + 7 * 24 * 60 * 60 * 1000); // 7 days from now
     
     // Check for events needing warning emails
+    // Skip events with null expires_at (they never expire)
     const eventsNeedingWarning = await db('events')
       .where('is_active', formatBoolean(true))
       .where('is_archived', formatBoolean(false))
+      .whereNotNull('expires_at')
       .where('expires_at', '<=', warningDate)
       .where('expires_at', '>', now);
     
@@ -40,9 +42,11 @@ async function checkExpirations() {
     }
     
     // Check for expired events
+    // Skip events with null expires_at (they never expire)
     const expiredEvents = await db('events')
       .where('is_active', formatBoolean(true))
       .where('is_archived', formatBoolean(false))
+      .whereNotNull('expires_at')
       .where('expires_at', '<=', now);
     
     for (const event of expiredEvents) {