Freeze an Order Item's Recipe at order time

Price was already snapshotted per Order Item so later menu changes
don't rewrite history, but the Recipe (Step list) wasn't: advancing an
Order Item looked up its next Step from the live menuItemSteps table,
so an operator editing a Menu Item's Recipe mid-cook could silently
reshape an Order already in flight (different Machine Tag, duration,
or Step order than what the customer paid for).

Adds order_item_steps as a per-Order-Item copy of the Recipe, taken at
placement time; placeOrder, advanceOrderItemStep, and getOrderStatus
now all read from that snapshot instead of the live menu. Recorded as
docs/adr/0005 and added Recipe as a formal CONTEXT.md term.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CONTEXT.md

@@ -30,6 +30,9 @@ _Avoid_: Line item (implies a billing-only concept; this also carries preparatio
 A bearer credential issued to a guest customer (one without an account) at Order placement time. Since a guest has no session, this token is the only proof of ownership accepted on later calls against that Order (status lookup, cancellation, pickup).
 _Avoid_: Session (guests never get a session; this is a different, bearer-credential mechanism)
 
+**Recipe**:
+The ordered list of Preparation Steps required to prepare a Menu Item. A Menu Item's Recipe can be edited later by an operator; an Order Item freezes a copy of the Recipe at the moment it's ordered, so a later Recipe edit never reshapes a Step list a customer already paid for.
+
 **Preparation Step**:
 One stage of preparing an Order Item, requiring a Machine carrying a specific Machine Tag, with an estimated duration. An Order Item consists of one or more Steps performed in sequence — the next Step cannot begin until the previous one finishes. The customer can see their Order Item's full Step list, current queue position, and approximate time to completion.
 _Avoid_: Stage (use Step consistently)