Add pre_mapped_vrs, pre_mapped_vrs_digest, and pre_mapped_vrs_version columns to Variant

[bencap]

Summary

Add three nullable columns to variants to hold the independently-minted pre-mapped VRS value. This issue is schema-only — no minting logic, wiring, or backfill.

Proposed behavior

• New Alembic revision, chained onto the current release-2026.3.0 head, adding to variants:
  • pre_mapped_vrs — JSONB, nullable.
  • pre_mapped_vrs_digest — String, nullable, indexed.
  • pre_mapped_vrs_version — String, nullable.
• No server_default on any of the three. Every existing row starts null; backfilling them is separate work.
• Update the Variant ORM model with matching Column declarations, placed alongside the existing hgvs_nt/hgvs_pro/hgvs_splice fields.

Acceptance criteria

• The migration applies cleanly on top of the current release-2026.3.0 head and downgrades cleanly, dropping all three columns.
• variants.pre_mapped_vrs_digest has a btree index.
• The Variant ORM model exposes pre_mapped_vrs, pre_mapped_vrs_digest, and pre_mapped_vrs_version, all nullable, defaulting to None.
• No existing row's data changes; no other table is read or written.
• A model-level test creates a Variant, sets all three new fields, commits, re-fetches by id, and asserts each value round-trips exactly, including the full JSONB structure of pre_mapped_vrs.

Implementation notes

• Model the migration file on the existing additional_mapping_columns revision, which adds columns to mapped_variants and target_genes the same way.
• Index pre_mapped_vrs_digest as a plain indexed string column, the same way clingen_allele_id is indexed on MappedVariant (Column(String, index=True, nullable=True)) — not as a JSONB expression index like mapped_variants' existing index on pre_mapped->>'id'. This column is meant to be queried and joined on directly.