Fix N+1 query in evolution chain serializer

[santichausis]

Problem

EvolutionChainDetailSerializer.build_chain_link_entry recurses over the evolution tree and, for every non-root species, ran its own PokemonEvolution.objects.filter(evolved_species=...) query with ~13 select_related joins. A linear 3-stage chain fires 2 of these queries; Eevee's 9-species chain fires 8.

Fix

Fetch all PokemonEvolution rows for the whole chain in a single query (filter(evolved_species__evolution_chain=obj)), group them in memory by evolved_species_id, and look them up from that dict during the recursion instead of querying per node. The query only runs when some species in the chain evolves from another, so single-species chains still issue no evolution query at all. The pk ordering (already applied by PokeApiManager) is spelled out explicitly at the call site.

Performance impact

Measured against the full dataset (DB built from the CSVs, all 540 evolution chains serialized with the old and new code):

• Output: byte-identical JSON for all 540 chains; OpenAPI schema unchanged.
• Queries: 1106 → 953 total (−14%). 134 chains need fewer queries (e.g. Eevee's 9-species chain: 9 → 2), none need more.
• The 65 queries for Milcery → Alcremie are not addressed here. They come from per-row lookups in PokemonEvolutionSerializer (condition_expression / allowed_natures), handled separately in Avoid per-row queries for evolution condition variables and natures #1690.
• In production, cachalot + Redis serves repeated queries from cache, so this mainly helps cold-cache requests.

Test plan

• test_evolution_chain_api_query_count_does_not_scale_with_chain_size: a 1-branch and a 6-branch chain issue the same number of queries (fails with 8 != 3 without the fix).
• test_evolution_chain_api_single_species_chain_skips_evolution_query: a single-species chain issues no PokemonEvolution query.
• test_evolution_chain_api_evolution_details_order_with_many_rows_for_same_species: evolution_details keeps creation order for a species with several evolution rows.
• manage.py test pokemon_v2 passes (66 tests); ruff check, ruff format --check and ty pass.

[pokeapi-machine-user]

A PokeAPI/api-data refresh has started. In ~45 minutes the staging branch of PokeAPI/api-data will be pushed with the new generated data.

The staging branch will be deployed in our staging environment and the entire API will be ready to review.

A Pull Request (master<-staging) will be also created at PokeAPI/api-data and assigned to the PokeAPI Core team to be reviewed. If approved and merged new data will soon be available worldwide at pokeapi.co.