Add metadata discovery endpoints for curves and exports

[louispt1]

Context

PyETM previously hard-coded the list of available curves and exports. Any new output required changes in pyetm too. These endpoints let pyetm discover available outputs at runtime so a new curve or export can be added to ETEngine without cascading changes.

Implemented changes

• Adds GET /api/v3/curves/metadata returning all registered hourly output curves with their name, type, and description
• Adds GET /api/v3/exports/metadata returning all registered annual exports with their name and description
• Introduces CurveMetadataRegistry, a registry populated at startup via an initialiser; curves and exports register themselves in their respective controllers

Related

Goes with this pyetm PR and this documentation PR

Checklist

• I have tested these changes
• I have updated documentation as needed
• I have tagged the relevant people for review