Importer perf: pre-fetch parent FK pk maps in create/update

[ajslater]

Summary

Implements tasks/importer-perf/02-create-fk-batching.md from PR #627. Second of the surgical N+1 wins, after PR #628.

_get_create_update_args previously called field_model.objects.get() once per row to dereference each parent FK reference. For a fresh 600k-comic import: ~2M small SELECTs across the imprint/series/volume/credit/identifier/storyarcnumber create and update phases, dominated by round-trip overhead.

The fix:

• New _build_parent_fk_pk_maps(model) walks MODEL_CREATE_ARGS_MAP[model] to find every parent FK model referenced, then pre-fetches a {key_tuple: pk} map per parent — one values_list("pk", *rels) query each.
• _get_create_update_args takes the prebuilt map and resolves parent references via dict lookup.
• The resolved pk is assigned via <field_name>_id so Django skips the FK-descriptor round-trip on construction. bulk_create / bulk_update write the same column either way; the update_fields / unique_fields tuples don't need to change since Django's ORM accepts both forms.

Net query count: ~2M → roughly K*2 where K is the number of distinct parent FK models (~12-20 SELECTs for a typical import). Python work unchanged.

Correctness notes

• Group-model parent slicing (values_tuple[:index+1] for Imprint/Series/Volume parents in GROUP_MODEL_COUNT_FIELDS) preserved exactly.
• Identifier-style multi-column key tuples preserved exactly.
• Nullable parent FKs (identifier): pk_map miss returns None, written as <field>_id=None — same as before.
• Required parent FK miss: previously DoesNotExist raised in the loop, now IntegrityError raised at bulk_create time. Both abort the import; only the exception class shifts. In practice, by the time create runs, the parent rows exist (created in a previous step or already in the DB), so this path is theoretical.

Test plan

• make fix clean
• make lint-python clean (0 errors, 0 warnings)
• pytest tests/importer/ tests/test_search_fts.py — 7 passed
• Field check on a real fixture: import a 1k-comic library twice (cold + warm) and a 1k-comic library with update_fields changes, assert resulting Publisher/Imprint/Series/Volume/Identifier/Credit/StoryArcNumber rows match the pre-PR baseline
• Wall-clock measurement on a real-scale fixture once one is available

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