feat(intake): swap a photo for a cut-out, keeping the original (#281)

Adds backend/src/intake/backgroundRemoval.ts, the shared module the drafting worker and the admin endpoints both call so a cut-out obtained either way is undoable the same way.

cutoutPathFor is pure and writes a new file beside the original rather than overwriting it, which is what keeps the original restorable and makes the JPEG-to-PNG change free. removeImageBackground only points the row at the new file after it is already on disk, and is idempotent via the original_image_path IS NOT NULL check — load-bearing twice, since it also stops a second pass from recording the cut-out as the original and losing the real one for good. restoreImageOriginal swaps the paths back and deliberately leaves the cut-out file on disk.

Extends the Task 1 integration test file with a stub sidecar bound to an ephemeral port and covers the no-op-on-repeat case plus three failure modes (500, non-image body, unreachable), asserting the row is left untouched in every failure case.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/unit/backgroundRemoval.test.ts

@@ -0,0 +1,22 @@
+import { cutoutPathFor } from '../../src/intake/backgroundRemoval';
+
+describe('where a cut-out is written', () => {
+  // A new file rather than a rewrite of the original, which is what makes the
+  // original restorable at all — and what makes the JPEG-to-PNG change free,
+  // since no existing path is renamed.
+  it('sits beside the original with a -cutout suffix and a .png extension', () => {
+    expect(cutoutPathFor('/uploads/abc-123.jpg')).toBe('/uploads/abc-123-cutout.png');
+  });
+
+  // image_path is a contract, not a string: #103 made the stored value the
+  // path uploadUrl joins an origin onto.
+  it('keeps the /uploads/ prefix', () => {
+    expect(cutoutPathFor('/uploads/x.webp')).toBe('/uploads/x-cutout.png');
+  });
+
+  // The extension is replaced rather than appended, so a second pass cannot
+  // produce `.png.png`.
+  it('replaces the extension rather than appending to it', () => {
+    expect(cutoutPathFor('/uploads/x.png')).toBe('/uploads/x-cutout.png');
+  });
+});