fix: standardize error response format

- Fix exchange.py: replace {'error':...} with HTTPException(detail=...)
- Fix payment/admin/teams/quotation: str(e) messages are already user-safe
- Confirm admin_search.py test endpoint uses correct probe pattern
- Confirm frontend has no raw alert() calls (already uses ElMessage)

backend/app/api/v1/exchange.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, HTTPException
 from app.services.exchange import ExchangeRateService
 from datetime import datetime
 
@@ -14,7 +14,7 @@ async def convert_currency(
 ):
     rate = await service.get_rate(from_currency, to_currency)
     if rate is None:
-        return {"error": f"No rate available for {from_currency} -> {to_currency}"}
+        raise HTTPException(status_code=404, detail=f"汇率不可用: {from_currency} → {to_currency}")
 
     return {
         "from_currency": from_currency.upper(),