[Bug] Signup endpoint returns JSON body with text/plain content-type header

[rohansx]

Bug

In app/modules/auth/auth_router.py, the POST /signup and POST /auth/custom-token endpoints use Response(content=json.dumps(...)) to return JSON data. The Response class defaults to Content-Type: text/plain, so clients that check the content-type header will not parse the body as JSON.

# Current (broken — Content-Type: text/plain):
return Response(content=json.dumps({"error": "Missing uid"}), status_code=400)

# Should be (Content-Type: application/json):
return JSONResponse(content={"error": "Missing uid"}, status_code=400)

18 instances across the signup handler and custom-token handler are affected. Meanwhile, other routes in the same file (login, SSO login, provider endpoints) correctly use JSONResponse.

Impact

• API clients and frontend code that checks the Content-Type header will not parse signup/custom-token responses as JSON
• Inconsistent response format within the same router — login returns application/json but signup returns text/plain for equivalent JSON bodies

Expected behavior

All endpoints returning JSON data should use JSONResponse (or set media_type="application/json") so the Content-Type header is correct.