Return 409 Conflict for duplicate user signup and map to inline frontend errors

[jameswhite-cmd]

Backend

• Add a standard APIError model (documented in OpenAPI) to describe error responses: {error, field, message}.
• Update POST /users and POST /users/signup to return HTTP 409 with a consistent JSON shape when an email is already in use (JSONResponse with {"error":"conflict","field":"email","message":"Already in use"}).
• Add OpenAPI response documentation for 409 on these endpoints.
• Update backend tests to expect 409 + the new error payload.

Frontend

• Update signup flow to accept ApiError and call setError when the API returns 409 conflict, mapping the APIError.field/message to an inline form error.
• Update Playwright test to attempt a duplicate signup and assert the inline message "Already in use" is visible.

Notes

• This makes duplicate signup handling consistent across the API surface and UX, and documents the error model in OpenAPI. Backend unit tests and the updated Playwright test were adjusted accordingly.

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This pull request was co-created with Cosine Genie

Original Task: full-stack-demo/4ob4p3l1v8ki
Author: James White