feat: comprehensive code review fixes + receivers-only architecture

Security:
- Refuse startup with default secret_key in production (was just logging)
- Settings endpoint now requires admin role
- Password validation on initial setup
- DOM-based HTML sanitizer replaces regex in template previews
- Add *.log to .gitignore

Performance & reliability:
- Token refresh deduplication prevents race condition on concurrent 401s
- Theme media query listener registered once (no leak)
- IconPicker uses $derived instead of function call per render
- Snackbar uses single-batch state update instead of while loop
- Replace 11 inline hover handlers with CSS :hover in layout

Architecture - receivers-only:
- Delivery endpoints (chat_id, email, url, room_id, topic) now stored
  exclusively in TargetReceiver rows, never in target.config
- Migration extracts existing delivery fields to receiver rows
- Notifier and dispatcher remove all config fallbacks
- Frontend targets page shows receivers list per target with
  add/remove/toggle/test per receiver
- Single-receiver test endpoint: POST /targets/{id}/receivers/{id}/test

Code quality:
- Extract AuthLayout.svelte from login/setup (150 lines CSS dedup)
- Split telegram-bots page (754→51 lines + 3 tab components)
- Split notification-trackers page (547→432 lines + 4 components)
- Deduplicate _send_reply into shared handler.send_reply()
- Add locale column to template models, replace name-based detection
- Fix delete_notification_tracker dead protection check
- Fix check_telegram_bot query (filter by type, remove bogus OR)
- Add graceful scheduler shutdown in lifespan
- Consistent /bots?tab=telegram URLs across all nav links

i18n:
- Error page, chat actions, target types, provider types internationalized
- All new receiver UI strings in EN + RU

frontend/src/routes/notification-trackers/TestMenu.svelte

@@ -0,0 +1,37 @@
+<script lang="ts">
+	import { t } from '$lib/i18n';
+	import MdiIcon from '$lib/components/MdiIcon.svelte';
+
+	interface Props {
+		testMenuOpen: string | null;
+		testMenuStyle: string;
+		ttTesting: Record<string, string>;
+		testTypes: { key: string; icon: string; labelKey: string }[];
+		ontest: (ttId: number, testType: string) => void;
+		onclose: () => void;
+	}
+
+	let { testMenuOpen, testMenuStyle, ttTesting, testTypes, ontest, onclose }: Props = $props();
+</script>
+
+{#if testMenuOpen}
+	<!-- svelte-ignore a11y_no_static_element_interactions -->
+	<div style="position:fixed; top:0; left:0; right:0; bottom:0; z-index:9998;"
+		onclick={onclose}
+		onkeydown={(e) => { if (e.key === 'Escape') onclose(); }}>
+	</div>
+	<div style="{testMenuStyle} background:var(--color-card); border:1px solid var(--color-border); border-radius:0.5rem; box-shadow:0 10px 25px rgba(0,0,0,0.3); padding:0.25rem; min-width:10rem;">
+		{#each testTypes as tt}
+			<button
+				onclick={() => ontest(Number(testMenuOpen), tt.key)}
+				disabled={!!ttTesting[`${testMenuOpen}_${tt.key}`]}
+				class="flex items-center gap-2 w-full px-3 py-1.5 text-sm rounded hover:bg-[var(--color-muted)] transition-colors disabled:opacity-50 text-left">
+				<MdiIcon name={tt.icon} size={14} />
+				{t(tt.labelKey)}
+				{#if ttTesting[`${testMenuOpen}_${tt.key}`]}
+					<span class="ml-auto text-xs text-[var(--color-muted-foreground)]">...</span>
+				{/if}
+			</button>
+		{/each}
+	</div>
+{/if}