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/plans.ts

@@ -0,0 +1,22 @@
+export const PLANS = ['free', 'plus'] as const;
+export type Plan = (typeof PLANS)[number];
+
+/**
+ * Every feature gate lives here so paid tiers can be reshaped without
+ * hunting through the codebase. Clients read these to show upsells.
+ */
+export interface Entitlements {
+  /** How often we refresh now-playing while someone is watching you. */
+  activePollSeconds: number;
+  maxConnections: number;
+  /** Friends you can pin to the top of your feed. */
+  maxPinnedFriends: number;
+  /** Placeholder for future paid features. */
+  listeningHistory: boolean;
+  profileThemes: boolean;
+}
+
+export const PLAN_ENTITLEMENTS: Record<Plan, Entitlements> = {
+  free: { activePollSeconds: 15, maxConnections: 3, maxPinnedFriends: 3, listeningHistory: false, profileThemes: false },
+  plus: { activePollSeconds: 5, maxConnections: 10, maxPinnedFriends: 20, listeningHistory: true, profileThemes: true },
+};