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>

backend/src/asyncRoute.ts

@@ -0,0 +1,20 @@
+import { Request, Response, NextFunction, RequestHandler } from 'express';
+
+// Express 4 does not forward a rejected promise from an async handler to the
+// error middleware. An async route that throws therefore never responds at all
+// — the request hangs until the client gives up, which reads to a user as "the
+// page is empty" rather than "the server failed". Wrapping the handler routes
+// the rejection into next(), so the error middleware can answer with a 500.
+//
+// Express 5 does this natively; drop this helper if the project ever upgrades.
+export function asyncRoute(
+  handler: (req: Request, res: Response, next: NextFunction) => unknown
+): RequestHandler {
+  return (req, res, next) => {
+    // Promise.resolve also captures a synchronous throw, so both failure modes
+    // reach the same place.
+    return Promise.resolve()
+      .then(() => handler(req, res, next))
+      .catch(next);
+  };
+}