Run a homogeneous batch of sprint-management writes.

627187b8 — the HTTP transport for meridian.db.batch_management's shared batch engine (86e4ae44), sharing meridian.batch_ops with the MCP tool execute_batch (meridian/mcp/handlers/sprint_tools.py) so both transports validate and execute identically. Body: {operation, entries, mode, idempotency_key, session_id?, max_entries?} — operation is one of sprint_items | item_updates | pointers | notes; mode and idempotency_key are REQUIRED keys (pass idempotency_key: null to explicitly opt out of idempotency protection). See execute_batch's MCP tool description (meridian/mcp_tools.py) for the full per-operation entry shape and response contract — this endpoint returns the exact same JSON shape.