feat: add news section with admin editor and public display

- NewsItem type with title, text, date, optional image and link
- Admin page at /admin/news with image upload and auto-date
- Public section between Pricing and FAQ, hidden when empty
- Nav link auto-hides when no news items exist

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/db.ts

@@ -295,6 +295,7 @@ const SECTION_KEYS = [
   "faq",
   "pricing",
   "schedule",
+  "news",
   "contact",
 ] as const;
 
@@ -325,6 +326,7 @@ export function getSiteContent(): SiteContent | null {
     faq: sections.faq,
     pricing: sections.pricing,
     schedule: sections.schedule,
+    news: sections.news ?? { title: "Новости", items: [] },
     contact: sections.contact,
     team: {
       title: teamSection.title || "",