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/routes/providers/+page.svelte

@@ -140,10 +140,12 @@
 					<input id="prv-name" bind:value={form.name} required class="flex-1 px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 				</div>
 			</div>
+			{#if form.type !== 'scheduler'}
 			<div>
 				<label for="prv-url" class="block text-sm font-medium mb-1">{t('providers.url')}</label>
 				<input id="prv-url" bind:value={form.url} required placeholder={form.type === 'gitea' ? 'https://gitea.example.com' : t('providers.urlPlaceholder')} class="w-full px-3 py-2 border border-[var(--color-border)] rounded-md text-sm bg-[var(--color-background)]" />
 			</div>
+			{/if}
 			{#if form.type === 'immich'}
 			<div>
 				<label for="prv-key" class="block text-sm font-medium mb-1">{editing ? t('providers.apiKeyKeep') : t('providers.apiKey')}</label>