feat(build): make the user-modpack build lane read its context (closes the last functional gap)

A submitted modpack was durable but unreadable: the uploads PVC cannot cross
namespaces (felis-api mounts it; Kaniko runs in felis-build) and the s3 lane
handed the sandboxed build Pod no credentials, so NO user build could ever
consume its context. The transport is now the API itself:

- submit: derived context refs become the internal-face URL
  /api/v1/internal/submissions/{id}/context (service-token gated), and Blobs
  gains Open (local + s3) with an ErrBlobNotFound sentinel for the route's 404.
- api: serves that route on the internal face only (openapi.yaml updated; the
  route-coverage test enforces it).
- build: an http(s) context renders a context-fetch initContainer (the felis
  image's new fetch-context entrypoint) that streams the blob with the
  namespace-local service-token Secret — never mounted into Kaniko — and
  extracts it under a zip-slip guard into a size-limited emptyDir that Kaniko
  reads read-only as --context=/context.
- platform/install: the api Deployment carries its own internal base URL; the
  build namespace gets the token Secret through the existing replica mechanism
  (bootstrap.sh + felis setup); the build egress lock opens exactly the control
  namespace on the internal port.
- cmd/felis: fetch-context entrypoint (registered, documented, unit-tested for
  escapes/symlinks/non-gzip).

Tests cover rendering, hardening, the s3/local Open paths, and the route's
404/503 mapping. Verified next on the real single-node cluster with Kaniko.

internal/platform/workloads.go

@@ -274,6 +274,10 @@ func APIDeployment(p Params) *appsv1.Deployment {
 			},
 		},
 		{Name: "FELIS_IMAGE", Value: p.FelisImage},
+		// The api's own internal-face base URL, so it derives the submission
+		// context URLs that build Pods fetch through it. Same value the login gate
+		// is handed; one address for one face.
+		{Name: naming.EnvAPIBaseURL, Value: InternalAPIBaseURL(p.ControlNamespace)},
 	}
 	if p.BackupPVC != "" {
 		env = append(env, corev1.EnvVar{Name: "FELIS_BACKUP_PVC", Value: p.BackupPVC})