Sample QC graphs - modernize (homozygosity / chrom density / allele frequency)

🤖 Written by Claude

*(Updated: the usage evidence originally cited here was mistakenly drawn from VG4 dev/test box logs. Real VG3 production logs (4–18 Aug 2026) show only ~5 views each of `homozygosity_graph`, `chrom_density_graph` and `sample_allele_frequency_histogram_graph` over 15 days — so this is **tech-debt driven, not usage-driven**. Reprioritise accordingly.)*

The sample QC graphs have had no substantive feature work since ~2022 (commit history is linting, the matplotlib modernization #1682, and refactors).

Ideas, if/when this area is worth touching:

* Rewrite `HomozygosityPercentGraph` to use the ORM — already flagged in #1465
* Move from server-side matplotlib + cached PNG + polling to interactive JSON graphs (as done for qc_graphs in #1120), which would also remove the `cached_generated_file/check` polling loop these pages generate
* On the VG4 test deployment the somalier ancestry/relatedness report links 404 (media files missing) — either fix generation/serving or hide the links when the files are absent

Begin by reviewing the approaches referenced in #1465 and #1120, then inspect the existing sample QC graph entry points and the cached_generated_file/check polling loop. Done requires a clearly chosen scope, updated homozygosity, chromosome-density, and allele-frequency graph behavior, and verified handling of the somalier report links when media files are absent.

Written by the indexing model from the issue text.