chore: migrate to pnpm workspaces monorepo with Turborepo

- Restructure directories: apps/api, apps/admin, apps/website
- Add root pnpm-workspace.yaml, turbo.json, .prettierrc, .gitignore
- Rename packages to @inkreach/api, @inkreach/admin, @inkreach/website
- Add shared packages: packages/tsconfig, packages/shared-types
- Add pnpm.onlyBuiltDependencies for native builds
- Update docs: README.md, structs.md
- All three projects build successfully

apps/api/src/auth/dto/login.dto.ts

@@ -0,0 +1,14 @@
+import { IsNotEmpty, IsString } from 'class-validator';
+
+/**
+ * Payload accepted by `POST /auth/login`.
+ */
+export class LoginDto {
+  @IsString()
+  @IsNotEmpty()
+  username!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  password!: string;
+}