fix: MEDIUM priority — shared validation, content caching, Schedule useReducer, stable keys

- Extract shared sanitization to src/lib/validation.ts, apply to all 3 registration routes (#2)
- Replace key={index} with stable keys in About and News (#4)
- Add 5-min in-memory content cache in content.ts, invalidate on admin section save (#6)
- Refactor Schedule from 8 useState calls to useReducer — single dispatch, fewer re-renders (#8)
- Remove Hero scroll indicator, add auto-scroll to next section on wheel/swipe

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/api/admin/sections/[key]/route.ts

@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import { getSection, setSection, SECTION_KEYS } from "@/lib/db";
 import { siteContent } from "@/data/content";
 import { revalidatePath } from "next/cache";
+import { invalidateContentCache } from "@/lib/content";
 
 type Params = { params: Promise<{ key: string }> };
 
@@ -34,6 +35,7 @@ export async function PUT(request: NextRequest, { params }: Params) {
 
   const data = await request.json();
   setSection(key, data);
+  invalidateContentCache();
   revalidatePath("/");
 
   return NextResponse.json({ ok: true });