Enforce stock, totals, status-transition and audit invariants purely with SQLite triggers

# Task: enforce shop invariants with SQLite triggers `schema.sql` defines products, orders, order_items and audit_log. Write **only** `triggers.sql` (it is executed right after `schema.sql` on an empty database with SQLite 3.46 via Python's `sqlite3`; do not change `schema.sql`). Application code will do plain INSERT/UPDATE/DELETE statements; the database itself must keep these rules: 1. Inserting an order item: if `unit_price_cents` is NULL it is set to the product's current `price_cents`; the product's `stock` decreases by `qty`. If stock is insufficient the statement fails with an error whose message contains `insufficient stock` and nothing changes. 2. Updating an item's `qty` adjusts stock by the difference (same insufficient-stock error). Changing an item's `product_id` is forbidden (error containing `immutable`). 3. Deleting an order item restores its stock. 4. `orders.total_cents` always equals the sum of `qty * unit_price_cents` over its items (0 when none). An item keeps the `unit_price_cents` it got at insert time; later product price changes do not affect existing items or totals. 5. Items can only be inserted, updated or deleted while their order's status is `pending`; otherwise error containing `order locked`. 6. Order status may only change pending->paid, pending->cancelled, paid->shipped, paid->cancelled (same status = no-op allowed). Any other change fails with error containing `bad transition`. Cancelling an order returns the stock of all its items (items stay, total stays). 7. Every change of a product's `price_cents` (old != new) writes one audit_log row: entity 'product', entity_id, field 'price_cents', old and new values as text. Every order status change writes a row with entity 'order', field 'status', old and new status. Updates that leave the value unchanged write no audit row; changes to other columns (e.g. `stock`) are not audited. 8. A product that appears in any order item cannot be deleted (error containing `in use`). Use `RAISE(ABORT, '...')` for errors. No network.