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/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "@musicshare/shared",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "scripts": {
+    "typecheck": "tsc -p ."
+  },
+  "dependencies": {
+    "zod": "^4.6.5"
+  }
+}