feat: admin panel with SQLite, auth, and calendar-style schedule editor

Complete admin panel for content management:
- SQLite database with better-sqlite3, seed script from content.ts
- Simple password auth with HMAC-signed cookies (Edge + Node compatible)
- 9 section editors: meta, hero, about, team, classes, schedule, pricing, FAQ, contact
- Team CRUD with image upload and drag reorder
- Schedule editor with Google Calendar-style visual timeline (colored blocks, overlap detection, click-to-add)
- All public components refactored to accept data props from DB (with fallback to static content)
- Middleware protecting /admin/* and /api/admin/* routes

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

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

@@ -0,0 +1,32 @@
+import { NextRequest, NextResponse } from "next/server";
+import { getSection, setSection, SECTION_KEYS } from "@/lib/db";
+import { revalidatePath } from "next/cache";
+
+type Params = { params: Promise<{ key: string }> };
+
+export async function GET(_request: NextRequest, { params }: Params) {
+  const { key } = await params;
+  if (!SECTION_KEYS.includes(key as typeof SECTION_KEYS[number])) {
+    return NextResponse.json({ error: "Invalid section key" }, { status: 400 });
+  }
+
+  const data = getSection(key);
+  if (!data) {
+    return NextResponse.json({ error: "Section not found" }, { status: 404 });
+  }
+
+  return NextResponse.json(data);
+}
+
+export async function PUT(request: NextRequest, { params }: Params) {
+  const { key } = await params;
+  if (!SECTION_KEYS.includes(key as typeof SECTION_KEYS[number])) {
+    return NextResponse.json({ error: "Invalid section key" }, { status: 400 });
+  }
+
+  const data = await request.json();
+  setSection(key, data);
+  revalidatePath("/");
+
+  return NextResponse.json({ ok: true });
+}