feat(billing): Polar.sh Pro subscription integration

- Webhook handler verifies HMAC-SHA256 sig and updates user role on
  subscription.created / subscription.updated / subscription.revoked
- Audio endpoint gated: free users limited to 3 chapters/day via Valkey
  counter; returns 402 {error:'pro_required'} when limit reached
- Translation proxy endpoint enforces 402 for non-pro users
- AudioPlayer.svelte surfaces 402 via onProRequired callback + upgrade banner
- Chapter page shows lock icon + upgrade prompts for gated translation langs
- Profile page: subscription section shows Pro badge + manage link (active)
  or monthly/annual checkout buttons (free); isPro resolved fresh from DB
- i18n: 13 new profile_subscription_* keys across all 5 locales

ui/src/routes/api/translation/[slug]/[n]/+server.ts

@@ -0,0 +1,69 @@
+import { error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { log } from '$lib/server/logger';
+import { backendFetch } from '$lib/server/scraper';
+
+const SUPPORTED_LANGS = new Set(['ru', 'id', 'pt', 'fr']);
+
+/**
+ * POST /api/translation/[slug]/[n]?lang=<lang>
+ * Proxy to backend translation enqueue endpoint.
+ * Enforces Pro gate — free users cannot enqueue translations.
+ *
+ * GET /api/translation/[slug]/[n]?lang=<lang>
+ * Proxy to backend translation fetch (no gate — already gated at page.server.ts).
+ */
+export const GET: RequestHandler = async ({ params, url }) => {
+	const { slug, n } = params;
+	const lang = url.searchParams.get('lang') ?? '';
+	const res = await backendFetch(
+		`/api/translation/${encodeURIComponent(slug)}/${n}?lang=${lang}`
+	);
+	if (!res.ok) {
+		return new Response(null, { status: res.status });
+	}
+	const data = await res.json();
+	return new Response(JSON.stringify(data), {
+		headers: { 'Content-Type': 'application/json' }
+	});
+};
+
+export const POST: RequestHandler = async ({ params, url, locals }) => {
+	const { slug, n } = params;
+	const chapter = parseInt(n, 10);
+	const lang = url.searchParams.get('lang') ?? '';
+
+	if (!slug || !chapter || chapter < 1) error(400, 'Invalid slug or chapter');
+	if (!SUPPORTED_LANGS.has(lang)) error(400, 'Unsupported language');
+
+	// ── Pro gate ──────────────────────────────────────────────────────────────
+	if (!locals.isPro) {
+		log.info('polar', 'translation blocked for free user', {
+			userId: locals.user?.id,
+			slug,
+			chapter,
+			lang
+		});
+		return new Response(
+			JSON.stringify({ error: 'pro_required' }),
+			{ status: 402, headers: { 'Content-Type': 'application/json' } }
+		);
+	}
+
+	const res = await backendFetch(
+		`/api/translation/${encodeURIComponent(slug)}/${chapter}?lang=${lang}`,
+		{ method: 'POST' }
+	);
+
+	if (!res.ok) {
+		const text = await res.text().catch(() => '');
+		log.error('translation', 'backend translation enqueue failed', { slug, chapter, lang, status: res.status, body: text });
+		error(res.status as Parameters<typeof error>[0], text || 'Translation enqueue failed');
+	}
+
+	const data = await res.json();
+	return new Response(JSON.stringify(data), {
+		status: res.status,
+		headers: { 'Content-Type': 'application/json' }
+	});
+};

ui/src/routes/api/translation/status/[slug]/[n]/+server.ts

@@ -0,0 +1,23 @@
+import type { RequestHandler } from './$types';
+import { backendFetch } from '$lib/server/scraper';
+
+/**
+ * GET /api/translation/status/[slug]/[n]?lang=<lang>
+ * Proxies the translation status check to the backend.
+ */
+export const GET: RequestHandler = async ({ params, url }) => {
+	const { slug, n } = params;
+	const lang = url.searchParams.get('lang') ?? '';
+	const res = await backendFetch(
+		`/api/translation/status/${encodeURIComponent(slug)}/${n}?lang=${lang}`
+	);
+	if (!res.ok) {
+		return new Response(JSON.stringify({ status: 'idle' }), {
+			headers: { 'Content-Type': 'application/json' }
+		});
+	}
+	const data = await res.json();
+	return new Response(JSON.stringify(data), {
+		headers: { 'Content-Type': 'application/json' }
+	});
+};