feat: scaffold monorepo with shared types and NestJS server

- pnpm workspace with packages/app, packages/server, packages/shared
- @mp-pilates/shared: enums, constants, TypeScript interfaces for all 8 data models
- @mp-pilates/server: NestJS bootstrap with health check, validation pipe, CORS
- Base TypeScript config with strict mode

tsconfig.base.json

@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "ES2021",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "resolveJsonModule": true,
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true,
+    "paths": {
+      "@mp-pilates/shared": ["./packages/shared/src/index.ts"],
+      "@mp-pilates/shared/*": ["./packages/shared/src/*"]
+    }
+  },
+  "exclude": ["node_modules", "dist"]
+}