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/settings/+page.svelte

@@ -7,10 +7,12 @@
 	import Loading from '$lib/components/Loading.svelte';
 	import MdiIcon from '$lib/components/MdiIcon.svelte';
 	import Hint from '$lib/components/Hint.svelte';
+	import ErrorBanner from '$lib/components/ErrorBanner.svelte';
 	import { snackSuccess, snackError } from '$lib/stores/snackbar.svelte';
 
 	let loaded = $state(false);
 	let saving = $state(false);
+	let error = $state('');
 	let settings = $state({
 		external_url: '',
 		telegram_webhook_secret: '',
@@ -20,16 +22,16 @@
 	onMount(async () => {
 		try {
 			settings = await api('/settings');
-		} catch (err: any) { snackError(err.message); }
+		} catch (err: any) { error = err.message; snackError(err.message); }
 		finally { loaded = true; }
 	});
 
 	async function save() {
-		saving = true;
+		saving = true; error = '';
 		try {
 			settings = await api('/settings', { method: 'PUT', body: JSON.stringify(settings) });
 			snackSuccess(t('settings.saved'));
-		} catch (err: any) { snackError(err.message); }
+		} catch (err: any) { error = err.message; snackError(err.message); }
 		saving = false;
 	}
 </script>
@@ -39,6 +41,7 @@
 {#if !loaded}
 	<Loading />
 {:else}
+	<ErrorBanner message={error} />
 	<div class="space-y-6">
 		<!-- General section -->
 		<Card>