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/api/api.go

@@ -271,6 +271,9 @@ func (a *API) internalAPIRoutes() []apiRoute {
 		{Method: "GET", Pattern: "/readyz", Public: true, h: a.handleReadyz},
 
 		{Method: "GET", Pattern: "/api/v1/servers", h: a.handleListServers},
+		// The build Pod's context-fetch initContainer streams a submission's stored
+		// modpack through this route (build namespace cannot mount the uploads PVC).
+		{Method: "GET", Pattern: "/api/v1/internal/submissions/{id}/context", h: a.handleInternalSubmissionContext},
 		{Method: "POST", Pattern: "/api/v1/internal/servers/{name}/ready", h: a.handleReady},
 		{Method: "POST", Pattern: "/api/v1/internal/servers/{name}/join-event", h: a.handleJoinEvent},
 		// Domain-autostart (spec §9.1, §14): velocity drives the wake lever and polls