Implement Dashboard CRUD API (GraphQL)

Description:
Implement GraphQL mutations and queries to allow users to create, read, update, and delete dashboards. Ensure the API supports:

• Optimistic locking to prevent overwrites (lock_version).
• Conflict resolution errors when another user has updated the dashboard.
• Proper RBAC enforcement according to dashboard permissions.

Acceptance Criteria:

• GraphQL queries: dashboard(id), dashboards(...) with pagination and optional search by name.
• GraphQL mutations: createDashboard, updateDashboard, deleteDashboard.
• Lock version conflict returns structured error with current server state.
• Permissions checks implemented for each CRUD operation.
• Unit and integration tests for all operations.