…n API

Add the kit's documented promise to the API-docs surface too: drop springdoc on the
classpath and /swagger-ui + /v3/api-docs come up with no wiring, the admin API grouped,
and a single property turns it off.

- OpenApiAutoConfiguration: @ConditionalOnClass(GroupedOpenApi) so it's dormant until the
  consumer adds springdoc (declared compileOnly here), @ConditionalOnWebApplication(SERVLET),
  and @ConditionalOnProperty(devslab.kit.openapi.enabled, matchIfMissing=true) so
  `devslab.kit.openapi.enabled=false` disables it without removing the dependency (typical
  in production). Contributes a GroupedOpenApi pinned to /admin/api/v1/** and a default
  OpenAPI document (title/version); both @ConditionalOnMissingBean so a consumer can override.
- DevslabKitProperties: new OpenApi block (enabled / admin-group / title / version).
- gradle.properties: SPRINGDOC_VERSION=3.0.3 — the Spring Boot 4 / Spring Framework 7 line
  (2.8.x targets Spring Boot 3; the Spring Boot BOM does not manage springdoc). Consumed
  compileOnly by autoconfigure.
- sample-app: add springdoc-openapi-starter-webmvc-ui (a consumer opting in) so the
  end-to-end path is exercised.

Verified by sample-app integration tests over the real HTTP stack — this is what proves
the springdoc-3.0.x-on-Jackson-3 concern is a non-issue for our setup:
- OpenApiEndpointTests (3): /v3/api-docs returns 200 + valid JSON (real serialization),
  the admin group /v3/api-docs/admin lists /admin/api/v1, and /swagger-ui/index.html is
  200 (not blocked — the admin security chain only matches /admin/api/v1/**).
- OpenApiDisabledTests (1): enabled=false removes the admin group.

Docs: README(.ko) Features + Configuration tables, docs/reference/configuration(.ko) new
OpenAPI section, CHANGELOG(.ko) Unreleased. Full `./gradlew build` green (44 tests, 0
failures); mkdocs --strict clean.