spec: update api consistency

Align optimistic locking conflict payload, update websocket docs, and adjust tests.

openspec/changes/update-api-consistency/specs/resource-management/spec.md

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+## MODIFIED Requirements
+### Requirement: Workload Heatmap UI
+The system SHALL provide a visual workload heatmap interface for managers.
+
+#### Scenario: View workload heatmap
+- **GIVEN** user is logged in as manager or admin
+- **WHEN** user navigates to /workload page without filters
+- **THEN** system displays a heatmap showing all accessible users' workload
+- **AND** users with zero workload are included by default
+- **AND** each user cell is color-coded by load level (green/yellow/red)
+
+#### Scenario: Hide empty workloads
+- **GIVEN** user is viewing the workload page
+- **WHEN** user enables the hide_empty filter
+- **THEN** the heatmap excludes users with zero workload
+
+#### Scenario: Navigate between weeks
+- **GIVEN** user is viewing the workload page
+- **WHEN** user clicks previous/next week buttons
+- **THEN** the heatmap updates to show that week's workload data
+
+#### Scenario: Default week window on Sunday
+- **GIVEN** today is Sunday and user opens workload page without selecting week_start
+- **THEN** the default heatmap window includes tasks due in the upcoming week
+
+#### Scenario: View user workload details
+- **GIVEN** user is viewing the workload heatmap
+- **WHEN** user clicks on a specific user's cell
+- **THEN** a modal/drawer opens showing that user's task breakdown
+- **AND** tasks show title, project, time estimate, and due date
+
+#### Scenario: Filter by department
+- **GIVEN** user is a system admin
+- **WHEN** user selects a department from the filter
+- **THEN** the heatmap shows only users from that department