feat: Actions system — scheduled mutations on external services

Full-stack implementation of provider-scoped Actions with extensible
executor architecture. First action type: Immich auto_organize (sort
assets into albums by person, CLIP search, date range, favorites).

Core:
- ActionTypeDefinition registry + ActionExecutor ABC with execute/validate/dry-run
- ImmichActionExecutor with multi-album support and client-side filtering
- ImmichClient write methods: add/remove assets, create album, paginated search

Server:
- Action, ActionRule, ActionExecution DB models
- Full CRUD API + manual execute + dry-run + execution history endpoints
- APScheduler integration (interval + cron) for automated execution
- Action type discovery API + provider people endpoint

Frontend:
- Actions page with CRUD, execute/dry-run buttons, inline rule editor
- RuleEditor: person/album MultiEntitySelect pickers, criteria config
- ExecutionHistory: expandable per-rule result details
- MultiEntitySelect reusable component (searchable multi-pick palette)
- Notification tracker album picker migrated to MultiEntitySelect
- Fixed MdiIcon race condition (icons missing after cache-clearing reload)

frontend/src/routes/+layout.svelte

@@ -84,6 +84,12 @@
 				{ href: '/command-template-configs', key: 'nav.templates', icon: 'mdiCodeBracesBox', countKey: 'command_template_configs' },
 			],
 		},
+		{
+			key: 'nav.automation', icon: 'mdiRobotOutline',
+			children: [
+				{ href: '/actions', key: 'nav.actions', icon: 'mdiPlayCircleOutline', countKey: 'actions' },
+			],
+		},
 		{
 			key: 'nav.bots', icon: 'mdiRobot',
 			children: [