Fix param validation errors and add TNS config section

- terceros/transaccion: return clear error when required params (doc_num, num_factura) are missing instead of passing empty dict to Firebird causing Column unknown error
- transaccion: remove prefijo from 'por factura' query; parse num_factura as int when digit
- automation: add /preview endpoint to show patient/reception counts before sending; update UI with preview step and disabled run button until preview loads
- config: add TNS credentials section (tns_empresa, tns_usuario, tns_password, api_sucursal) with test-connection button
- config: add POST /config/test-tns endpoint

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routes/config.py

@@ -1,7 +1,8 @@
 from fastapi import APIRouter, Request, Form, Depends
-from fastapi.responses import RedirectResponse
+from fastapi.responses import JSONResponse, RedirectResponse
 from app.database import get_connection
 from app.auth import get_current_user
+from app.services.api_client import get_tns_token
 
 router = APIRouter(prefix="/config", tags=["config"])
 
@@ -45,6 +46,20 @@ async def config_page(request: Request, user: dict = Depends(get_current_user)):
     })
 
 
+@router.post("/test-tns")
+async def test_tns(
+    request: Request,
+    user: dict = Depends(get_current_user),
+    empresa: str = Form(""),
+    usuario: str = Form(""),
+    password: str = Form(""),
+):
+    token, err = await get_tns_token(empresa, usuario, password)
+    if token:
+        return JSONResponse({"success": True, "message": "Login TNS exitoso"})
+    return JSONResponse({"success": False, "message": err or "Error desconocido"})
+
+
 @router.post("/save")
 async def config_save(request: Request, user: dict = Depends(get_current_user)):
     form = await request.form()