refactor: comprehensive codebase review — security, performance, quality, UX

Security:
- Fix NUT protocol command injection (validate names against safe regex)
- Enable Jinja2 autoescape=True to prevent HTML injection via external data
- Add WebhookProviderConfig validation model

Performance:
- Shared aiohttp.ClientSession singleton (replaces 40+ per-request sessions)
- Fix 4 N+1 queries with batch IN loads (poller, scheduler, memory, broadcast)
- asyncio.gather for Gitea commands and notification dispatcher
- Add DB indexes on NotificationTrackerState.tracker_id, CommandTrackerListener
- LRU cache for compiled Jinja2 templates
- Daily EventLog cleanup job (90-day retention)
- 30s HTTP timeout on all external calls
- GROUP BY for target type counts (replaces 7 sequential queries)

Code quality:
- Extract get_owned_entity() helper (replaces 11 duplicate functions)
- Extract slot_helpers.py (load_slots, save_slots, render_template_preview)
- Extract command_utils.py (tracker lookup, last event, collection IDs)
- Extract http_session.py (shared session lifecycle)
- Provider connection validation dedup (3x → 1 helper)
- Command dispatch tables replacing if/elif chains
- Album+links fetch helper (fetch_albums_with_links)
- Provider dispatch polymorphism (list_provider_collections)
- Immutable _enrich_assets (no longer mutates in-place)
- Fix _format_assets return type + handler unpacking

Frontend:
- Fix 18+ hardcoded English strings → t() with new i18n keys (en + ru)
- Mobile "More" nav panel with provider filter and search
- Shared Button.svelte component (4 variants, 2 sizes)
- Shared ErrorBanner.svelte component (8 pages updated)
- SvelteKit goto() replacing window.location.href
- Dashboard grid fixed for 4 cards, paginator opacity consistency

Functionality:
- max_instances=1 on scheduler jobs (prevents duplicate events)
- Webhook provider in watcher (prevents error spam)
- Fix stale SQLModel reference in poller
- Gitea get_repo() direct API call

frontend/src/routes/command-configs/+page.svelte

@@ -15,6 +15,7 @@
 	import IconGridSelect from '$lib/components/IconGridSelect.svelte';
 	import { providerTypeItems, providerTypeFilterItems, responseModeItems } from '$lib/grid-items';
 	import EntitySelect from '$lib/components/EntitySelect.svelte';
+	import Button from '$lib/components/Button.svelte';
 	import { snackSuccess, snackError } from '$lib/stores/snackbar.svelte';
 	import { highlightFromUrl } from '$lib/highlight';
 	import { globalProviderFilter } from '$lib/stores/provider-filter.svelte';
@@ -37,7 +38,7 @@
 	let cmdTemplateConfigs = $derived(commandTemplateConfigsCache.items);
 	const templateItems = $derived(cmdTemplateConfigs
 		.filter((c) => c.provider_type === form.provider_type)
-		.map((c) => ({ value: c.id, label: c.name + (c.user_id === 0 ? ' (System)' : ''), icon: c.icon || 'mdiCodeBracesBox', desc: c.provider_type }))
+		.map((c) => ({ value: c.id, label: c.name + (c.user_id === 0 ? t('common.systemSuffix') : ''), icon: c.icon || 'mdiCodeBracesBox', desc: c.provider_type }))
 	);
 	let loaded = $state(false);
 	let showForm = $state(false);
@@ -151,10 +152,9 @@
 </script>
 
 <PageHeader title={t('commandConfig.title')} description={t('commandConfig.description')}>
-	<button onclick={() => { showForm ? (showForm = false, editing = null) : openNew(); }}
-		class="px-3 py-1.5 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90">
+	<Button size="sm" onclick={() => { showForm ? (showForm = false, editing = null) : openNew(); }}>
 		{showForm ? t('common.cancel') : t('commandConfig.newConfig')}
-	</button>
+	</Button>
 </PageHeader>
 
 {#if !loaded}<Loading />{:else}