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