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>

Dockerfile

@@ -22,4 +22,12 @@ COPY --from=backend-build /app/backend/migrations ./migrations
 COPY --from=frontend-build /app/frontend/dist ./public
 ENV NODE_ENV=production
 EXPOSE 3000
-CMD ["node", "dist/server.js"]
+# Migrations run before the app serves, so deployed code can never be ahead of
+# the database schema. Previously this was a separate, manual
+# `docker exec ... node migrate.js up` that was easy to forget — and forgetting
+# it meant every item query referenced tables that did not exist yet, which
+# surfaced as an empty storefront rather than an error.
+#
+# `&&` matters: migrate.js exits non-zero on failure, so a bad migration stops
+# the container instead of letting it serve against a half-migrated schema.
+CMD ["sh", "-c", "node migrate.js up && node dist/server.js"]