Define backend spec for dashboard features

[lg-epitech]

Once the dashboard design in #14 is finalized, define the backend feature specification needed to support it.

This should identify:

• which parts of the backend are already covered by Teams CRUD endpoints #12
• which additional tables, endpoints, and computed data are still needed
• the contract required by the frontend for each dashboard section

The spec should cover the dashboard areas shown in the design:

• summary metrics for open tickets, dispatched tickets, failed dispatches, and resolved tickets over the past week
• recent dispatch activity feed
• teams overview
• integrations and system health indicators

The output should be a clear backend spec that can be used to implement the remaining dashboard support without revisiting the design phase.

[linear-code]

DIS-18