Add backfill script to link subgenre Tags to parent genres

[Chisomnwa]

Add backfill script for genre refs on subgenre Tags

Summary

Adds scripts/backfill_genre_refs.py to populate the genre field on our 24 existing subgenre Tags, linking each subgenre to its parent genre Tag(s). This is step 2 of the Genre Explorer phasing plan (issue #13158).

What this does

Reads the parent genre mapping from our subgenre vocabulary (tag_types/subgenres/vocabulary.json), matches each subgenre to its parent genre Tag key from the genre vocabulary, and writes the genre reference to each subgenre Tag object on Open Library.

Example

Biopunk (/tags/OL268T) will get:

"genre": [{"key": "/tags/OL179T"}]

Linking it to Sci-Fi (/tags/OL179T).

Gothic (/tags/OL281T) will get:

"genre": [{"key": "/tags/OL171T"}, {"key": "/tags/OL177T"}, {"key": "/tags/OL174T"}]

Linking it to Horror, Romance, and Literary.

Usage

python scripts/backfill_genre_refs.py --dry-run   # preview only
python scripts/backfill_genre_refs.py              # write changes

Dry-run results:

• Loaded 24 subgenre-parent mappings from vocabulary
• Loaded 21 genre mappings from vocabulary
• Done: 24 subgenre Tags updated, 0 errors

Why we load genre keys from vocabulary, not OL search API

We initially tried querying the OL search API (/search.json) to discover genre Tags, but Tags aren't indexed in Solr yet (@cdrini hasn't indexed them). The search API returns 0 results for Tags.

But, the script doesn't need Solr indexing at all — it uses the OL JSON API (via olclient) to fetch and save Tags, not the search API.

We load genre keys from the vocabulary files instead of querying OL because:

• It's simpler — the vocabulary already has every genre Tag key we created in Phase 2A
• No external dependency — no network calls for the lookup
• Solr is irrelevant here — Solr only matters later, when the Genre Explorer needs to search/query the genre→subgenre hierarchy

When Tags get indexed in Solr, the search-based approach can be used as an alternative.

Prerequisites

The PR adding genre property to /type/tag must be merged first PR link
~/.config/ol.ini must have bot credentials (for non-dry-run)
Usage

Reviewrs

@mekarpeles