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

docker-compose.yml

@@ -0,0 +1,17 @@
+services:
+  postgres:
+    image: postgres:17
+    environment:
+      POSTGRES_USER: musicshare
+      POSTGRES_PASSWORD: musicshare
+      POSTGRES_DB: musicshare
+    ports:
+      - "${POSTGRES_PORT:-5447}:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+  redis:
+    image: redis:7
+    ports:
+      - "${REDIS_PORT:-6380}:6379"
+volumes:
+  pgdata: