Handle curves and exports via the metadata endpoint

[louispt1]

Context

Fetch curves from the added metadata endpoints in etengine, with a fallback for version 2025-01. This way available curves and exports are discovered via the endpoint and cached, and then that information is re-used when fetching outputs.

Implemented changes

• Adds CurveMetadataService — fetches /curves/metadata and /exports/metadata from ETEngine at runtime, with in-memory TTL cache, disk cache keyed by base URL, and hardcoded fallback defaults for stable version use
• Adds CurveRegistry with canonical (new name) / wire (on-wire name) split so curve requests remain compatible with 2025-01 engines that only know old names (e.g. merit_order, not electricity_profiles)
• Replaces hard-coded export and curve name lists throughout validators.py, annual_exports.py, hourly_output_curves.py, and excel_utils.py

Related

Goes with this etengine 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