fix(compose): keep the scene background instead of losing it on refresh

[catomean]

The bug

The scene background — the place the figures stand in — was held only in a URL.createObjectURL blob in React state. It was never uploaded and compositions.background_url was never written, even though the column, the backgrounds/ storage folder, and an authorized, versioned /api/compositions route all already existed. Nothing in the app called that route.

Two consequences:

1. Reopen the project and the canvas is bare black — the background is gone.
2. The export silently ships that. handleExportPng renders the Konva stage as it finds it, so it produces panel files with no scene in them and still reports Print-ready — N panel files at 200 DPI. Same failure shape as the baked-in guides in fix(export): produce a file the print shop can actually use #20: a confident success message on an artifact nobody re-checks before it reaches the print shop.

The fix

• useComposition / useSaveComposition (matching the useSurface idiom) wire the existing endpoint up.
• Background upload goes to storage (backgrounds/, already supported by getStoragePath; insert is still permitted after fix(storage): take delete and update away from the anon key #24) and the path to the composition row, so it reloads with the project.
• Loaded via crossOrigin = 'anonymous', the same as FigureLayer — that is what keeps the stage untainted so the export can still read pixels back out.
• The image is derived from the saved path rather than stored, so a stale background can't be painted (and no setState in an effect).
• Export and the background button are disabled while a save is in flight; exporting then would print the bare canvas.

Only background_url is written. Figure placement already lives on the figures rows — copying it into layout would give it a second home.

Regression guard

An orphaned endpoint is what made this invisible. wired.test.ts fails if any route family has no caller in the app. Mutation-verified: removing the new hook fails it on /api/compositions, and it passes with the hook restored.

Verify

• lint: 0 errors, 8 warnings (all pre-existing)
• tsc: clean
• tests: 78 pass · 0 fail (was 73)

🤖 Generated with Claude Code

https://claude.ai/code/session_014opKWKa65PXxn2MiWSKwwh