Set up npm workspaces monorepo with Turborepo and local infra

Postgres and Redis run via docker compose on non-default ports (5447/6380)
to avoid clashing with other local services. React is pinned to a single
version across workspaces because Expo requires it.

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

turbo.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "tasks": {
+    "dev": {
+      "cache": false,
+      "persistent": true
+    },
+    "build": {
+      "dependsOn": ["^build"],
+      "outputs": [".next/**", "!.next/cache/**", "dist/**"]
+    },
+    "typecheck": {
+      "dependsOn": ["^typecheck"]
+    }
+  }
+}