Add shared package: types, schemas, plans and API client

Holds everything web, mobile and API must agree on: DTOs, zod input
schemas, plan entitlements (the single place feature gates live),
streaming-service link helpers, feed ordering and a typed API client.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

packages/shared/src/time.ts

@@ -0,0 +1,7 @@
+export function timeAgo(iso: string): string {
+  const s = Math.max(0, (Date.now() - new Date(iso).getTime()) / 1000);
+  if (s < 60) return 'just now';
+  if (s < 3600) return `${Math.floor(s / 60)}m ago`;
+  if (s < 86400) return `${Math.floor(s / 3600)}h ago`;
+  return `${Math.floor(s / 86400)}d ago`;
+}