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>

README.md

@@ -160,4 +160,6 @@ Gitea Actions runs two workflows on every push to `main` and on pull requests:
 
 ## Production deployment
 
-Production runs as a single Docker image (multi-stage build — the frontend is built to static files and served directly by the backend), deployed via Portainer behind Nginx Proxy Manager, with authentik forward-auth gating `/admin`. That infrastructure is homelab-specific and documented separately outside this repo.
+Production runs as a single Docker image (multi-stage build — the frontend is built to static files and served directly by the backend), deployed via Portainer behind Nginx Proxy Manager, with authentik forward-auth gating `/admin`. That infrastructure is homelab-specific and documented separately outside this repo.
+
+The container applies pending migrations before starting the server, so deployed code can never be ahead of the database schema. A failed migration stops the container rather than letting it serve against a schema it doesn't match — check `docker logs` on the app container if it doesn't come up.