[moonlight16]

Part of the work split out of rossoctl/moca#455 (the original context-delegation PR). Ports the upload and freeze work from #50 (the original upload/freeze PR, kept open as a reference) onto main after #51 (removed sandbox pools).

Moca delivers context to its shared, long-running sandboxes as a bundle. This PR adds what it needs:

Call	What it does
POST /internal/v1/contexts	Moca creates a Context owned by the user it acts for
POST {ctx}/upload-capabilities	One-time upload URL and token for the client (5 minutes, single use)
PUT /v1/uploads/{id}	The client uploads a .context bundle; a short-lived helper Pod validates it and writes a revision
POST /internal{ctx}/freeze	Pins that exact revision; later uploads are rejected
GET /internal{ctx}/revisions/{sha}/bundle	New: streams the frozen revision back to Moca
DELETE /internal{ctx}	Deletes the Context

{ctx} is /v1/namespaces/{ns}/contexts/{name}. Trusted calls require Moca's service token (CS_CONTROL_PLANE_TOKEN) and the user Moca acts for (X-Context-Subject). docs/context-upload.md has the flow diagram and the limits.

Differences from #50:

• Left out the PVC attachment endpoint and the PVC name in responses. Bundle delivery doesn't need them; per-workload sandboxes that mount PVCs are planned in /workloads phase 2: give each workload its own sandboxes and, optionally, durable storage moca#476 (phase 2 workloads).
• A failed export aborts the response instead of returning a truncated 200.
• Context Service starts without a helper image; only upload and export are then unavailable.

Known limits:

• Upload capabilities live in memory, so run one replica.
• Freeze blocks uploads and new revisions, but not snapshot restore, clone, or direct PVC sync. Follow-up.

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