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/command-trackers/+page.svelte

@@ -275,13 +275,9 @@
 
 						<!-- Add listener -->
 						<div class="flex items-center gap-2 mt-2">
-							<select bind:value={newListenerBotId[trk.id]}
-								class="flex-1 px-2 py-1 text-xs border border-[var(--color-border)] rounded-md bg-[var(--color-background)]">
-								<option value={0} disabled selected>{t('commandTracker.selectBot')}</option>
-								{#each telegramBots as bot}
-									<option value={bot.id}>{bot.name} {bot.bot_username ? `(@${bot.bot_username})` : ''}</option>
-								{/each}
-							</select>
+							<div class="flex-1">
+								<EntitySelect items={botItems} bind:value={newListenerBotId[trk.id]} placeholder={t('commandTracker.selectBot')} />
+							</div>
 							<button onclick={() => addListener(trk.id)} disabled={!newListenerBotId[trk.id] || addingListener[trk.id]}
 								class="text-xs px-3 py-1 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md hover:opacity-90 disabled:opacity-50">
 								{addingListener[trk.id] ? t('common.loading') : t('commandTracker.addListener')}