feat: add Scheduler provider + multi-provider UX fixes

Scheduler provider:
- Virtual provider (no external service) that emits SCHEDULED_MESSAGE
  events on user-defined intervals or cron expressions
- Custom variables stored in tracker filters, flattened into template context
- fire_count persists across triggers via tracker state
- APScheduler CronTrigger support for cron-mode schedules
- Default templates (EN+RU), seeded on startup

Multi-provider UX fixes:
- Tracking config hides Immich-specific sections (periodic, scheduled,
  memory, asset display) for non-Immich providers
- Command config driven by provider capabilities — hides commands/settings
  for providers without bot commands
- Template config hides empty "Scheduled Messages" group
- Test menu on tracker targets is provider-aware (Immich shows all 4 test
  types, others show only basic)
- Removed redundant Test button from tracker card
- System-owned tracking configs (user_id=0) seeded for Gitea + Scheduler
- Fixed ownership checks to allow system configs in tracker-target links
- Capabilities cache shared across template-configs and command-configs
- Command tracker bot selector uses EntitySelect instead of raw select
- Sample context includes Gitea + Scheduler variables for template preview

frontend/src/lib/grid-items.ts

@@ -100,4 +100,5 @@ export const previewTargetTypeItems = (): GridItem[] => [
 export const providerTypeItems = (): GridItem[] => [
 	{ value: 'immich', icon: 'mdiCamera', label: t('providers.typeImmich') },
 	{ value: 'gitea', icon: 'mdiGit', label: t('providers.typeGitea') },
+	{ value: 'scheduler', icon: 'mdiClockOutline', label: t('providers.typeScheduler') },
 ];