fix: run migrations on boot and stop failures rendering as empty (#23)

The storefront showed no inventory after deploying the categories/tags
release. No data was lost: the code queried categories/item_tags/
items.category_id against a database where the migration had not been
run, and that failure was invisible at every layer.

Three changes, each addressing one layer:

Migrations now run at container start, so deployed code cannot be ahead
of the schema and the easily-forgotten manual `docker exec migrate.js
up` step disappears. migrate.js waits for Postgres to accept
connections first, since the NAS brings the DB container up slower than
the app, and still exits non-zero so a bad migration stops the
container rather than serving a half-migrated schema.

Express 4 does not forward a rejected async handler, and no error
middleware was mounted, so a failing query never responded at all. Async
routes are now wrapped and an error middleware guarantees a 500. A hung
request is indistinguishable from an empty result in the UI, which is
how a schema mismatch came to read as "the store has no items".

The storefront now separates "request failed" from "no items" and offers
a retry. fetchItems/fetchFilterOptions throw on a non-OK response rather
than returning the parsed error body, which would have been set as the
item list and crashed the grid on .map.

Also restores the project-context update from 7fb5764, which was left
out of PR #24 and ended up dangling.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/api.ts

@@ -54,11 +54,16 @@ export async function fetchConfig(): Promise<SiteConfig> {
 export async function fetchItems(filters?: ItemFilters): Promise<Item[]> {
   const query = filters ? filtersToSearchParams(filters).toString() : '';
   const res = await fetch(query ? `/api/items?${query}` : '/api/items');
+  // An error response still parses as JSON — as `{ error: ... }`, not an array.
+  // Returning that unchecked would set it as the item list and crash the grid
+  // on `.map`, so a failure has to surface as a rejection the caller can show.
+  if (!res.ok) throw new Error('failed to load items');
   return res.json();
 }
 
 export async function fetchFilterOptions(): Promise<FilterOptions> {
   const res = await fetch('/api/filters');
+  if (!res.ok) throw new Error('failed to load filters');
   return res.json();
 }