Add unit and integration test suites

- Shared: link building, feed ordering, input schemas, API client
- API unit: track matching, private-address blocking, token crypto,
  plan expiry, provider response parsing
- API integration against Postgres and Redis: auth, follows and pins,
  now-playing state, webhooks, OAuth return URLs, realtime, rate limits
- Tests use a dedicated *_test database and Redis DB, and refuse to wipe
  anything else; fetch is stubbed so they never hit the network
- Pin Node to 22.13+ (.nvmrc), which Vite's native bindings require

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

packages/shared/package.json

@@ -7,9 +7,14 @@
     ".": "./src/index.ts"
   },
   "scripts": {
-    "typecheck": "tsc -p ."
+    "typecheck": "tsc -p .",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "zod": "^4.6.5"
+  },
+  "devDependencies": {
+    "vitest": "^5.0.3"
   }
 }