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

frontend/src/features/settings/hooks/useSettingsState.ts

@@ -49,6 +49,8 @@ export interface EventSettings {
   event_require_customer_name: boolean;
   event_require_customer_email: boolean;
   event_require_admin_email: boolean;
+  event_require_event_date: boolean;
+  event_require_expiration: boolean;
 }
 
 export function useSettingsState() {
@@ -109,7 +111,9 @@ export function useSettingsState() {
   const [eventSettings, setEventSettings] = useState<EventSettings>({
     event_require_customer_name: true,
     event_require_customer_email: true,
-    event_require_admin_email: true
+    event_require_admin_email: true,
+    event_require_event_date: true,
+    event_require_expiration: true
   });
 
   // Account form state
@@ -178,7 +182,9 @@ export function useSettingsState() {
       setEventSettings({
         event_require_customer_name: toBoolean(settings.event_require_customer_name, true),
         event_require_customer_email: toBoolean(settings.event_require_customer_email, true),
-        event_require_admin_email: toBoolean(settings.event_require_admin_email, true)
+        event_require_admin_email: toBoolean(settings.event_require_admin_email, true),
+        event_require_event_date: toBoolean(settings.event_require_event_date, true),
+        event_require_expiration: toBoolean(settings.event_require_expiration, true)
       });
     }
   }, [settings, i18n]);

frontend/src/features/settings/tabs/EventsTab.tsx

@@ -99,6 +99,56 @@ export const EventsTab: React.FC<EventsTabProps> = ({
               </div>
             </label>
           </div>
+
+          <div>
+            <label className="flex items-start gap-3">
+              <input
+                type="checkbox"
+                checked={eventSettings.event_require_event_date}
+                onChange={(e) => setEventSettings(prev => ({ ...prev, event_require_event_date: e.target.checked }))}
+                className="mt-1 w-4 h-4 text-primary-600 rounded focus:ring-primary-500"
+              />
+              <div>
+                <span className="text-sm font-medium text-neutral-700">
+                  {t('settings.events.requireEventDate', 'Require event date')}
+                </span>
+                <p className="text-xs text-neutral-500 mt-1">
+                  {t('settings.events.requireEventDateHelp', 'Event date must be provided when creating events')}
+                </p>
+                {!eventSettings.event_require_event_date && (
+                  <p className="text-xs text-amber-600 mt-1 flex items-center gap-1">
+                    <AlertCircle className="w-3 h-3" />
+                    {t('settings.events.eventDateWarning', 'Gallery URLs will use random identifiers instead of dates')}
+                  </p>
+                )}
+              </div>
+            </label>
+          </div>
+
+          <div>
+            <label className="flex items-start gap-3">
+              <input
+                type="checkbox"
+                checked={eventSettings.event_require_expiration}
+                onChange={(e) => setEventSettings(prev => ({ ...prev, event_require_expiration: e.target.checked }))}
+                className="mt-1 w-4 h-4 text-primary-600 rounded focus:ring-primary-500"
+              />
+              <div>
+                <span className="text-sm font-medium text-neutral-700">
+                  {t('settings.events.requireExpiration', 'Require expiration date')}
+                </span>
+                <p className="text-xs text-neutral-500 mt-1">
+                  {t('settings.events.requireExpirationHelp', 'Galleries must have an expiration date')}
+                </p>
+                {!eventSettings.event_require_expiration && (
+                  <p className="text-xs text-amber-600 mt-1 flex items-center gap-1">
+                    <AlertCircle className="w-3 h-3" />
+                    {t('settings.events.expirationWarning', 'Galleries without expiration will remain active until manually archived')}
+                  </p>
+                )}
+              </div>
+            </label>
+          </div>
         </div>
 
         <div className="mt-6">