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/+page.svelte

@@ -231,7 +231,7 @@
 		</div>
 	</Card>
 {:else if status}
-	<div class="grid grid-cols-1 sm:grid-cols-3 gap-4 mb-8 stagger-children">
+	<div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-4 gap-4 mb-8 stagger-children">
 		{#each statCards as card, i}
 			<div class="stat-card" style="--accent: {card.color};">
 				<div class="stat-card-inner">
@@ -289,7 +289,7 @@
 		<div class="flex items-center justify-center gap-1">
 			{#if totalPages > 1}
 				<button onclick={() => goToPage(currentPage - 1)} disabled={currentPage <= 1}
-					class="px-2 py-1 text-sm border border-[var(--color-border)] rounded-md hover:bg-[var(--color-muted)] transition-colors disabled:opacity-30 disabled:cursor-default">
+					class="px-2 py-1 text-sm border border-[var(--color-border)] rounded-md hover:bg-[var(--color-muted)] transition-colors disabled:opacity-50 disabled:cursor-default">
 					<MdiIcon name="mdiChevronLeft" size={16} />
 				</button>
 				{#each Array.from({ length: totalPages }, (_, i) => i + 1) as page}
@@ -305,7 +305,7 @@
 					{/if}
 				{/each}
 				<button onclick={() => goToPage(currentPage + 1)} disabled={currentPage >= totalPages}
-					class="px-2 py-1 text-sm border border-[var(--color-border)] rounded-md hover:bg-[var(--color-muted)] transition-colors disabled:opacity-30 disabled:cursor-default">
+					class="px-2 py-1 text-sm border border-[var(--color-border)] rounded-md hover:bg-[var(--color-muted)] transition-colors disabled:opacity-50 disabled:cursor-default">
 					<MdiIcon name="mdiChevronRight" size={16} />
 				</button>
 			{/if}