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.

deploy/bootstrap.sh

@@ -2148,6 +2148,10 @@ deploy_bundle() {
     --from-file=felis.toml="${STATE_DIR}/felis.pod.toml" \
     --dry-run=client -o yaml | kube apply -f -
   apply_literal_secret "$CONTROL_NS" felis-service-token token "$SERVICE_TOKEN"
+  # The build namespace needs the same token: the build Job's fetch initContainer
+  # streams a submission's build context from the felis-api internal face, and a
+  # secretKeyRef is namespace-local (a PVC cannot carry it across either).
+  apply_literal_secret "$BUILD_NS" felis-service-token token "$SERVICE_TOKEN"
   # The forwarding key every backend verifies the proxy's handshake with. `felis setup`
   # replicates it into the minecraft namespace (ensureSecretReplica) before it creates
   # the pods that mount it; the operator injects it into EVERY backend, because Velocity's