refactor: rename idp→serve and applyapi→gateway.api

[iAlexeze]

The applyapi package and idp schema key were named for the initial proof-of-concept. Both have since grown into a full self-service delivery surface — token scoping, target mode, response shaping, label/annotation fields, per-CRD permissions — and the original names no longer reflect what the feature is.

This rename makes the surface legible at a glance:

• gateway.applyAPI → gateway.api — the block that enables the Gateway API
• pkg/gateway/applyapi → pkg/gateway/api — the package that implements it
• idp: → serve: on CRD entries — the per-CRD self-service declaration
• ork idp → ork serve — the CLI subcommand

No logic changed. Types, field names, and behaviour are identical — only identifiers moved.

What changed

• All pkg/types/types_idp* → types_serve*
• All pkg/katalog/validate_idp* → validate_serve*
• pkg/gateway/applyapi/ → pkg/gateway/api/
• cmd/cli/idp.go → cmd/cli/serve.go
• Fixture testdata, doc file renames, reference schema, CHANGELOG, README, concepts pages updated to match