Fix Relay generating null query text for mutations

Relay v20 treats extend type in server schema as client extensions,
causing all mutation fields to have "text": null in generated artifacts.

Replace the cat-based schema concatenation with a merge script that
collects all extend type Mutation blocks and produces a single
type Mutation definition. The split graphql/ files remain the source
of truth for gqlgen; the merged schema.graphql is generated for Relay.

Signed-off-by: Émile Ré <emile@getprobo.com>

.gitignore

@@ -18,4 +18,5 @@ compose/keycloak/probo-realm.json
 # Generated files (codegen)
 __generated__/
 pkg/server/api/*/v1/types/types.go
+pkg/server/api/*/v1/schema.graphql
 cfg/dev_local.yaml