Resolve metadata drift between accession-based and sequence-based targets

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Overview

TargetAccession and TargetSequence have diverged significantly in their metadata representations. Accession-based targets carry user-supplied free-text fields (assembly, gene) with no validation against the actual accession, and have no structured organism/taxonomy representation despite the organism being fully deterministic from the accession. Sequence-based targets carry a full Taxonomy FK, but it is stored on the wrong model — taxonomy is a property of the gene target, not of the sequence representation.

This parent tracks two child issues that together resolve the drift:

• Remove assembly and gene from TargetAccession; derive from accession during mapping #696
• Move taxonomy_id to TargetGene; populate via mapping for accession-based targets #697

These issues are sequenced: #697 depends on #696 because the mapping job changes in #696 establish the derivation pattern that #697 extends to taxonomy. We also have created #698 to track API response normalization that must be coordinated with clients at a later time.