Return 409 Conflict for duplicate users and map to signup form error

[jameswhite-cmd]

What I changed

• Backend

  • Added a ConflictError model to app/models.py to document a consistent error response shape used for conflicts: {"error":"conflict","field":"email","message":"Already in use"}.
  • Modified the create_user and register_user handlers in app/api/routes/users.py to defensively return a 409 JSONResponse when an existing email is detected, and declared the 409 response in the route OpenAPI metadata (responses={409: {"model": ConflictError}}).
  • This provides a stable, documented API surface for duplicate email/username conflicts.

• Tests

  • Updated backend unit tests (backend/tests/api/routes/test_users.py) to expect HTTP 409 and the new JSON error body for duplicate user creation / signup cases.

• Frontend

  • Updated signup flow (frontend/src/routes/signup.tsx) to import and handle the ApiError shape, map an API 409 conflict to an inline form error via setError(field), and fall back to the generic handleError for other failures.
  • Updated the Playwright test (frontend/tests/sign-up.spec.ts) to assert the inline error text "Already in use" is visible when attempting to sign up with an existing email.

Why

• The API now consistently communicates duplicate resource conflicts with HTTP 409 and a structured JSON body so frontend clients can map the error directly to form fields.
• The OpenAPI model (ConflictError) documents the error response so clients and consumers can rely on the shape.
• Tests were updated to reflect the new behavior and ensure regression coverage.

Files changed (high level)

• backend/app/api/routes/users.py: return JSONResponse 409 for duplicate email, add responses metadata
• backend/app/models.py: add ConflictError model
• backend/tests/api/routes/test_users.py: updated assertions to expect 409 and new body
• frontend/src/routes/signup.tsx: map 409 conflict to setError for inline field error
• frontend/tests/sign-up.spec.ts: expect inline "Already in use" message

Notes

• This PR focuses on returning a consistent conflict response and wiring it through frontend handling and tests. Database unique constraints and any additional defensive checks should be verified/added in subsequent commits if not already present in the DB schema migrations.

---

This pull request was co-created with Cosine Genie

Original Task: full-stack-demo/uqvx8xon2xmt
Author: James White