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/services/events.service.ts

@@ -20,15 +20,15 @@ const normalizeEvent = (event: Event): Event => {
 interface CreateEventData {
   event_type: string;
   event_name: string;
-  event_date: string;
+  event_date?: string;
   customer_name?: string;
-  customer_email: string;
-  admin_email: string;
+  customer_email?: string;
+  admin_email?: string;
   require_password?: boolean;
   password?: string;
   welcome_message?: string;
   color_theme?: string;
-  expiration_days: number;
+  expiration_days?: number;
   allow_user_uploads?: boolean;
   upload_category_id?: number | null;
   feedback_enabled?: boolean;