Add catalog model layers with JSON Schema based kind declarations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

.changeset/catalog-backend-model-processor.md

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+---
+'@backstage/plugin-catalog-backend': minor
+---
+
+Added `ModelProcessor` that validates catalog entities against the compiled catalog model schemas, and integrated it into the `CatalogBuilder` and `CatalogPlugin`.

.changeset/catalog-model-layers.md

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+---
+'@backstage/catalog-model': minor
+---
+
+Added a new catalog model layer system that allows plugins to declare and extend catalog entity kinds, annotations, labels, tags, and relations using JSON Schema. The new `createCatalogModelLayer` API provides a builder for composing model definitions, and a `compileCatalogModel` function validates and merges them into a unified model. Built-in entity kinds now include model layer definitions.

.changeset/catalog-node-model-registry.md

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+---
+'@backstage/plugin-catalog-node': minor
+---
+
+Added `provideStaticCatalogModel` that helps provide a static catalog model at startup.

.changeset/scaffolder-backend-model-registration.md

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+---
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+Integrated the Template kind model layer with the catalog model registry service.

.changeset/scaffolder-common-template-model.md

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+---
+'@backstage/plugin-scaffolder-common': minor
+---
+
+Added a Template kind model layer with JSON Schema definition, serving as an example of how plugins can declare their own catalog entity kinds.