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]);