feat(orders): add amendment history visibility (#512)

[TYDev01]

closed #512
feat(orders): add amendment history visibility (#512)

Persist amendment metadata on swap orders and expose it in the UI so
users can see how many times an order changed and what the latest state
is.

Backend:

• Add amendment_count (int) and amendment_log (JSONB) columns to
  swap_orders with migration 20260601_0005
• Add PATCH /orders/{id}/amend endpoint: validates the order is open,
  diffs changed fields (from_amount, to_amount, min_fill_amount, expiry),
  appends a structured log entry with sequence, amended_at, per-field
  before/after, and optional note, then increments amendment_count
• Extend OrderResponse with amendment_count and amendment_log
• Add OrderAmend and OrderAmendmentEntry Pydantic schemas
• Emit order.updated WebSocket event on every amendment
• Add tests: OrderAmend schema validation and amendment log accumulation

Frontend:

• Add OrderAmendmentEntry type and amendmentLog field to Order
• Extend ApiOrderRecord and Zod schema with amendment fields
• Add amendOrder() API helper (PATCH)
• List view: show amendment count badge on each card
• Detail card: show "Last amended" timestamp in Order Summary panel
• Expandable AmendmentHistory panel with per-entry sequence number,
  timestamp, field-level before→after diffs, note, and "Latest" badge
• Populate amendmentLog in mock seed data for amended orders

Closes #512

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