feat(build)/docs: make executor images configurable; document the build lane's real seams (#9, #10)

- [registry] gains kaniko_image / trivy_image / build_cpu_limit /
  build_mem_limit overrides; empty keeps the compiled-in defaults. An
  air-gapped or mirrored install has no route to gcr.io/aquasec (the
  build egress policy allows only DNS + registry + package mirrors), so
  builds previously could not even start their executors.
- deferred-seams: the uploads-context entry now records WHY a mount is
  impossible (PVCs cannot cross namespaces) and that the s3 lane also
  lacks credentials in the build Pod — options captured for the real fix.
- troubleshooting 8e (executor ImagePullBackOff + the overrides),
  13b rewritten (verified eviction refusal, 5m pressure-transition,
  image-GC recovery), 15 (upgrade/rollback runbook for Recreate).
- Backup semantics decided and documented: a backup is the whole /data
  volume (worlds + config + plugins + cache) and a restore rolls all of
  it back — OpenAPI/README wording updated to match (same-tag images are
  still watched for regressions by the openapi parity gate).

cmd/felis/api.go

@@ -402,6 +402,14 @@ func buildConfig(cfg *config.Config) build.Config {
 	return build.Config{
 		Namespace:   cfg.Registry.BuildNamespace,
 		RegistryURL: cfg.Registry.URL,
+		// Empty overrides fall back to the build package's defaults, so an
+		// install that has not imported kaniko/trivy keeps the compiled-in refs
+		// (and fails loudly on pull rather than silently building with the wrong
+		// image).
+		KanikoImage: cfg.Registry.KanikoImage,
+		TrivyImage:  cfg.Registry.TrivyImage,
+		CPULimit:    cfg.Registry.BuildCPULimit,
+		MemLimit:    cfg.Registry.BuildMemLimit,
 	}
 }