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/lib/components/MdiIcon.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { getMdiPath } from '$lib/mdi-lookup';
+	import { getMdiPath } from '$lib/mdi-lookup.svelte';
 
 	let { name = '', size = 18 } = $props<{ name: string; size?: number }>();
 </script>