feat(showcase): per-line Receipt attachment on invoice lines (upload-in-grid demo) (#3051)

Adds a `receipt: Field.file()` to showcase_invoice_line so the showcase
exercises objectui#2360 end-to-end: the inline line-item grid auto-derives
an upload column and renders a compact picker per row (the "attach the
receipt to each expense line" pattern). Verified in-browser against this
example: upload → save via /api/v1/batch → value persists with a canonical
/api/v1/storage/files/<id> URL → reopening the form renders the stored chip.


Claude-Session: https://claude.ai/code/session_01H5tsBSEhsDZmdcHBvfymYC

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-zhuang

examples/app-showcase/src/data/objects/invoice.object.ts

@@ -210,6 +210,11 @@ export const InvoiceLine = ObjectSchema.create({
       min: 0,
       readonlyWhen: P`parent.status == 'paid'`,
     }),
+    // Per-line attachment (objectui#2360): the inline line-item grid renders a
+    // real upload cell for `file` fields — the "attach the receipt to each
+    // expense line" pattern. Exercises upload-in-grid end-to-end (auto-derived
+    // column, compact picker, persistence through the storage service).
+    receipt: Field.file({ label: 'Receipt' }),
     // Amount = Qty × Unit Price. Kept as a *stored* currency column (so the
     // parent Invoice.total summary can roll it up — summary aggregation reads
     // stored columns, not on-read formula fields), but the `expression` makes