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/template-configs/+page.svelte

@@ -3,7 +3,7 @@
 	import { slide } from 'svelte/transition';
 	import { api } from '$lib/api';
 	import { t } from '$lib/i18n';
-	import { templateConfigsCache } from '$lib/stores/caches.svelte';
+	import { templateConfigsCache, capabilitiesCache } from '$lib/stores/caches.svelte';
 	import PageHeader from '$lib/components/PageHeader.svelte';
 	import Card from '$lib/components/Card.svelte';
 	import Loading from '$lib/components/Loading.svelte';
@@ -109,8 +109,8 @@
 	let form = $state(defaultForm());
 	let previewTargetType = $state('telegram');
 
-	// Provider capabilities: loaded dynamically
-	let allCapabilities = $state<Record<string, any>>({});
+	// Provider capabilities: from shared cache
+	let allCapabilities = $derived(capabilitiesCache.items);
 	let providerTypes = $derived(Object.keys(allCapabilities));
 
 	// Dynamic slot definitions based on selected provider_type
@@ -137,10 +137,10 @@
 	onMount(load);
 	async function load() {
 		try {
-			[, varsRef, allCapabilities] = await Promise.all([
+			[, varsRef] = await Promise.all([
 				templateConfigsCache.fetch(true),
 				api('/template-configs/variables'),
-				api('/providers/capabilities'),
+				capabilitiesCache.fetch(),
 			]);
 		} catch (err: any) { error = err.message || t('common.loadError'); snackError(error); }
 		finally { loaded = true; highlightFromUrl(); }
@@ -284,7 +284,7 @@
 			<IconGridSelect items={previewTargetTypeItems()} bind:value={previewTargetType} columns={2} />
 		</div>
 
-		{#each templateSlots as group}
+		{#each templateSlots.filter(g => g.slots.length > 0) as group}
 			<fieldset class="border border-[var(--color-border)] rounded-md p-3">
 				<legend class="text-sm font-medium px-1">{t(`templateConfig.${group.group}`)}{#if group.group === 'eventMessages'}<Hint text={t('hints.eventMessages')} />{:else if group.group === 'scheduledMessages'}<Hint text={t('hints.scheduledMessages')} />{/if}</legend>
 				<div class="space-y-3 mt-2">