Create Cluster

Upsert the user's cluster snapshot.

Called by Clarity MCP server after successful clustering. Single-row model: INSERT if no snapshot exists, UPDATE if one does.

Headers: X-User-ID: User UUID (required) X-Internal-Secret: Internal API secret (required in production)

Body: cluster_data: {"clusters": [...]} thoughts_exhausted: bool correctly_clustered: bool | null

Returns: 201: {"cluster_id": "...", "created_at": "..."} 400: Invalid cluster_data structure 401: Missing or malformed X-User-ID header 404: User not found

Security: Validates X-Internal-Secret header to ensure request comes from trusted MCP server. In dev mode (INTERNAL_API_SECRET empty), this check is skipped.