refactor: reorganize admin GraphQL schema into modular structure
Split client_schema.gleam (2060 lines) into 5 logical modules:
- graphql/admin/types.gleam - Object types + get_field() helper
- graphql/admin/converters.gleam - Domain-to-GraphQL transformations
- graphql/admin/queries.gleam - Query resolvers
- graphql/admin/mutations.gleam - Mutation resolvers
- graphql/admin/schema.gleam - Public build_schema entry point
Additional improvements:
- Add TimeRange Gleam type with parser for type-safe enum handling
- DRY up activityBuckets resolver using the new type
- Rename handler from client_graphql to admin_graphql
- Remove redundant is_admin wrapper functions
- Fix import consistency in converters module