feat(doctrine): eagerly load translations of translatable resources

[paullla]

Q	A
Branch?	master
Bug fix?	yes (the #10 N+1, open since 2019)
New feature?	yes, TranslationsEagerLoadingExtension (experimental) + eager_load_translations option
BC breaks?	no (responses unchanged; only the generated SQL differs; kill switch provided)
Deprecations?	no
Issues	Fix #10
License	MIT

Problem

Reading a translated field resolves the locale through matching() on the EXTRA_LAZY translations collection, which issues one query per entity per locale: listing N resources costs N extra queries (#10).

Changes

• New TranslationsEagerLoadingExtension (experimental): fetch-joins the translations of TranslatableInterface resources on collection and item queries, so the collections come back initialized and matching() filters in memory.
• All locales are joined, not only the current one: a locale-constrained fetch join would mark the collection initialized with a subset, breaking fallback-locale reads and the translations serialization group. Locale-constrained joins added by filters are ignored by the guard for the same reason, and get a dedicated unconstrained join next to them.
• New eager_load_translations option (default true); set false to restore lazy loading.
• The service is registered only when the API Platform Doctrine ORM extension interfaces exist (api-platform/symfony does not require the doctrine-orm bridge); tag priorities mirror core's EagerLoadingExtension (collection -18, item -8).
• README (config block, Limitations), CHANGELOG and UPGRADE-2.1 entries.

Verification

• 100 unit/functional tests green, 8 new: produced DQL for collection and item, hydrated collections are initialized, non-translatable resources and the disable switch are no-ops, no duplicate join on double application, coexistence with a locale-constrained filter join. PHPStan level 6 and php-cs-fixer clean.
• Verified in demo apps (API Platform 4.3 and 3.4) over real HTTP with a query-logging DBAL middleware: listing 6 books costs 8-9 queries with eager loading off and 3 flat with it on (Doctrine Paginator count + ids + fetch join), responses byte-identical in both modes; item GET runs in 1 query; ?locale=, ?groups[]=translations, PATCH merge and fallback reads unchanged; invalid config rejected at compile time.