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_test.go

@@ -179,6 +179,11 @@ func TestAPIDeployment_Wiring(t *testing.T) {
 	if v := envValue(c.Env, "FELIS_IMAGE"); v != p.FelisImage {
 		t.Errorf("FELIS_IMAGE = %q, want %q", v, p.FelisImage)
 	}
+	// The internal-face base URL the api derives submission context-fetch URLs
+	// from — the same address the login gate is handed.
+	if v := envValue(c.Env, "FELIS_API_BASE_URL"); v != InternalAPIBaseURL(p.ControlNamespace) {
+		t.Errorf("FELIS_API_BASE_URL = %q, want %q", v, InternalAPIBaseURL(p.ControlNamespace))
+	}
 	// FELIS_SERVICE_TOKEN must come from a Secret, never a literal value.
 	tok := envVar(c.Env, "FELIS_SERVICE_TOKEN")
 	if tok == nil || tok.ValueFrom == nil || tok.ValueFrom.SecretKeyRef == nil {