feat(intake): add upload_links and item_drafts, and default an item's price (#222)

The schema for the intake pipeline. A submission becomes an `items` row at status 'pending' — already invisible to every public and storefront query since #90 — with an `item_drafts` row beside it holding the submitter's note, which link it arrived through, and the fields the drafting worker will fill in later.

`upload_links` stores a digest rather than a token, so a leaked database is not also a leaked set of working links, and the admin screen can show a token exactly once. `max_submissions` is nullable for "no cap", but the route will default it to a finite number: an unbounded link should be something asked for, not something that happens when nobody thought about it.

`item_drafts.upload_link_id` is ON DELETE SET NULL rather than CASCADE. Deleting a link must not delete the items that arrived through it — provenance is lost, the goods are not.

`items.price_cents` keeps NOT NULL and gains a default of 80.00, so an arriving item is always priced. That is the decision taken in the design review over making the column nullable: it costs the schema-level guarantee that nothing can publish at a price nobody chose, and buys not having to teach the cart, the checkout and thirteen other files about an item without a price. The protection moves into the review queue, and `price_source` exists so that queue can say whether a number came from a model, the default, or a person.

The number lives in the migration rather than in configuration. Changing a default price is a rare, deliberate act that deserves a record; an environment variable would let it drift silently between environments, and a wrong default is invisible until something has already sold at it.

Verified up, down and up again rather than only forwards — an irreversible migration is one that cannot be tested. Then verified by inspection rather than assumption: the default reads 8000, both tables and the state index exist, and an item inserted with no price comes back at 8000.

Backend: 263 integration, 302 unit, all passing against the new schema.

Ref #222

backend/tests/integration/setup/testDb.ts

@@ -28,9 +28,9 @@ export async function migrate(): Promise<void> {
 
 export async function resetDb(): Promise<void> {
   await testPool.query(`
-    TRUNCATE TABLE orders, checkout_items, checkouts, shipping_addresses, cart_items, carts,
-      customer_tokens, customer_sessions, favorites, customers, item_tags, item_images, items,
-      tags, categories
+    TRUNCATE TABLE item_drafts, upload_links, orders, checkout_items, checkouts,
+      shipping_addresses, cart_items, carts, customer_tokens, customer_sessions, favorites,
+      customers, item_tags, item_images, items, tags, categories
     RESTART IDENTITY CASCADE
   `);