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/providers/+page.svelte

@@ -12,12 +12,14 @@
 	import EmptyState from '$lib/components/EmptyState.svelte';
 	import ConfirmModal from '$lib/components/ConfirmModal.svelte';
 	import IconButton from '$lib/components/IconButton.svelte';
+	import ErrorBanner from '$lib/components/ErrorBanner.svelte';
 	import IconGridSelect from '$lib/components/IconGridSelect.svelte';
 	import { providerTypeItems, providerDefaultIcon } from '$lib/grid-items';
 	import { globalProviderFilter } from '$lib/stores/provider-filter.svelte';
 	import { snackSuccess, snackError } from '$lib/stores/snackbar.svelte';
 	import { highlightFromUrl } from '$lib/highlight';
 	import { getDescriptor, buildProviderFormDefaults } from '$lib/providers';
+	import Button from '$lib/components/Button.svelte';
 	import type { ServiceProvider } from '$lib/types';
 
 	let allProviders = $derived(providersCache.items);
@@ -136,10 +138,9 @@
 </script>
 
 <PageHeader title={t('providers.title')} description={t('providers.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('providers.cancel') : t('providers.addProvider')}
-	</button>
+	</Button>
 </PageHeader>
 
 {#if !loaded}
@@ -158,9 +159,7 @@
 {#if showForm}
 	<div in:slide={{ duration: 200 }}>
 	<Card class="mb-6">
-		{#if error}
-			<div class="bg-[var(--color-error-bg)] text-[var(--color-error-fg)] text-sm rounded-md p-3 mb-4">{error}</div>
-		{/if}
+		<ErrorBanner message={error} />
 		<form onsubmit={save} class="space-y-3">
 			<div>
 				<label class="block text-sm font-medium mb-1">{t('providers.type')}</label>
@@ -211,10 +210,9 @@
 				<p class="text-xs text-[var(--color-muted-foreground)] mt-1">{t('providers.webhookUrlHint')}</p>
 			</div>
 			{/if}
-			<button type="submit" disabled={submitting}
-				class="px-4 py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90 disabled:opacity-50">
+			<Button type="submit" disabled={submitting}>
 				{submitting ? t('providers.connecting') : (editing ? t('common.save') : t('providers.addProvider'))}
-			</button>
+			</Button>
 		</form>
 	</Card>
 	</div>