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.

docs/openapi.yaml

@@ -607,6 +607,34 @@ paths:
         '404':
           $ref: '#/components/responses/NotFound'
 
+  /api/v1/internal/submissions/{id}/context:
+    get:
+      tags: [submissions-internal]
+      operationId: internalSubmissionContext
+      summary: Stream a submission's stored build-context tarball to the build Pod.
+      description: >-
+        The build Job's fetch initContainer cannot mount the control-plane uploads
+        PVC (a PVC does not cross namespaces) and holds no object-store
+        credentials, so the API that stored the blob streams it here. Served on
+        the internal face (service token, no Zero Trust).
+      x-felis-face: [internal]
+      x-felis-tier: service
+      security: [{ serviceToken: [] }]
+      parameters:
+        - { name: id, in: path, required: true, schema: { type: string } }
+      responses:
+        '200':
+          description: The stored gzip tarball, verbatim.
+          content:
+            application/gzip:
+              schema: { type: string, format: binary }
+        '401':
+          $ref: '#/components/responses/Unauthorized'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '503':
+          $ref: '#/components/responses/ServiceUnavailable'
+
   /api/v1/internal/servers/{name}/join-event:
     post:
       tags: [servers-internal]