refactor: Rename Variant → AssayedVariant and MappedVariant → MappingRecord

[bencap]

Context

⚠️ Deferred — do not schedule until the DerivedAllele feature is stable.

The current naming obscures the semantic role of each entity. With DerivedAllele added, the full chain is:

• What the assay measured → Variant (clearer as: AssayedVariant)
• How it maps to a reference → MappedVariant (clearer as: MappedAllele)
• What nucleotide variants it implies → DerivedAllele

Goal

Rename Variant → AssayedVariant and MappedVariant → MappedAllele across the API codebase as a single atomic refactor. This is a large surface area change — coordinate any breaking response field renames with UI and API consumers before executing.

Scope

• SQLAlchemy model renames + Alembic migration for any table/column renames
• All FK references updated
• Router, endpoint parameter, and response schema renames
• API response field names updated (breaking change — coordinate with UI and consumers)
• All tests updated
• OpenAPI spec updated