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/notification-trackers/+page.svelte

@@ -17,6 +17,8 @@
 	import { providerDefaultIcon } from '$lib/grid-items';
 	import { globalProviderFilter } from '$lib/stores/provider-filter.svelte';
 	import { getDescriptor } from '$lib/providers';
+	import Button from '$lib/components/Button.svelte';
+	import ErrorBanner from '$lib/components/ErrorBanner.svelte';
 	import type { Tracker, TrackerTarget, TrackingConfig, TemplateConfig, NotificationTarget } from '$lib/types';
 
 	import TrackerForm from './TrackerForm.svelte';
@@ -119,7 +121,7 @@
 				capabilitiesCache.fetch(),
 			]);
 		} catch (err: any) {
-			loadError = err.message || 'Failed to load data';
+			loadError = err.message || t('common.loadFailed');
 			snackError(loadError);
 		} finally { loaded = true; highlightFromUrl(); }
 	}
@@ -212,7 +214,7 @@
 				}
 			}
 		}
-		if (created > 0) snackSuccess(`Created ${created} public link(s)`);
+		if (created > 0) snackSuccess(t('notificationTracker.createdLinks').replace('{count}', String(created)));
 		linkWarning = null;
 		linkCreating = false;
 		await doSave();
@@ -361,17 +363,16 @@
 </script>
 
 <PageHeader title={t('notificationTracker.title')} description={t('notificationTracker.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('notificationTracker.cancel') : t('notificationTracker.newTracker')}
-	</button>
+	</Button>
 </PageHeader>
 
 {#if !loaded}
 <Loading />
 {:else if loadError}
 	<Card>
-		<div class="bg-[var(--color-error-bg)] text-[var(--color-error-fg)] text-sm rounded-md p-3">{loadError}</div>
+		<ErrorBanner message={loadError} class="mb-0" />
 	</Card>
 {:else if showForm}
 	<TrackerForm