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/middleware/auth.js

@@ -168,9 +168,10 @@ async function galleryAuth(req, res, next) {
       return res.status(404).json({ error: 'Gallery not found or expired' });
     }
     
-    // Check if gallery has expired
-    if (new Date(event.expires_at) < new Date()) {
-      return res.status(410).json({ 
+    // Check if gallery has expired (only if expires_at is set)
+    // Galleries with null expires_at never expire
+    if (event.expires_at && new Date(event.expires_at) < new Date()) {
+      return res.status(410).json({
         error: 'Gallery has expired',
         code: 'GALLERY_EXPIRED'
       });